Before properly analyzing the balance, I think it important to note some secondary concerns that are unrelated to balance.
First, I think a modified skill list would make sense from a flavor perspective. The two Knowledge skills stand out in that respect - I think replacing them with Survival and Tumble would make the most sense. Survival would make sense for a Barbarian style class, and Tumble would synergize well with the increased mobility from Tiger Claw.
Second, I'm noticing that there is no mental stat synergy. Everything requiring Charisma is gone, and the three disciplines on offer all key off of Strength and Dexterity. While there's nothing inherently wrong with this, and the Berserker is clearly built to be a big dumb brute, I think the way the Tome of Battle encourages martial characters to use their mental stats is one of the more underrated achievements of the book. Some built-in synergy with Survival or Intimidate would fit the flavor without being too centralizing.
Anyway, as for the balance itself...
Perhaps stronger at low levels, but definitely weaker later on.
Low Levels
At very low levels, I don't think you'll be especially likely to trigger Berserker Strength very often. Often by the time an enemy rolls well enough to hit a meaningful attack, the fight is already half-over. That's okay, though, since you've still got the vast majority of what makes a low-level Crusader so powerful, plus the added power of things like Wall of Blades and Sudden Leap. Nothing the Berserker gives up is really all that important to a low-level Crusader, since Indomitable Soul and Zealous Surge don't see a lot of use until mid levels.
So, at low levels, I think there's a slight edge to the Berserker thanks to Tiger Claw having such exceptional low level stuff. There is a much greater advantage in the specific case of a glass cannon build - I could see a Wood Elf using the elven penalty to Con to live in a near-constant state of rage, for instance. This build would fall off hard at later levels, of course, but it could be viable enough to warrant some kind of clause stating that Berserker Strength can't kick in at full HP.
Mid Levels
At this point, Steely Resolve is really starting to catch up and give the Berserker the temporary survivability needed to make the most of Berserker Strength. Unfortunately, this is also when in-combat healing is really falling off. Packing 25 damage in a delayed damage pool is a lot more dangerous when there's no way to heal yourself, too. Level 11 is a godsend for this class in that respect, as it's both when Berserker Strength levels up and when the party healer learns Heal.
This is around the point where you're going to really start missing Indomitable Soul and Zealous Surge, though. The bonus on saves from Berserker Strength certainly helps, but it doesn't do nearly enough to cover the loss fully. While Iron Heart Surge lends a hand with this problem, the Crusader's random maneuver selection holds it back a lot.
It's not all bad, of course. Tiger Claw is giving a lot of useful battlefield mobility, and the fact that so many Tiger Claw maneuvers have overlapping functions is actually incredibly useful in the context of the Crusader's randomness. The class is also giving some incredibly serious damage output when things line up right. I would not personally find this a suitable upside for the marked loss in defenses, but that's entirely a matter of opinion at this point.
High Levels
This is where spells are really flying and damage has crossed the threshold of "anything less than 3 digits may as well be a peashooter." That said, the berserker is definitely keeping up on damage. High levels are surely going to give many ways to cheese the Berserker into a perpetually low health pool to boot.
However, even if we assume perpetual rage, this class has had to give up a lot of utility, and it's going to be felt here. This class gives up the opportunity to double-dip Charisma into will saves, gives up a safety net of rerolls against domination spells, foregoes the potent team synergy of White Raven and Devoted Spirit, and locks itself completely into the role of face-smasher by relying on rage.
Yes, it smashes lots of face. But, at high levels, that's a really low bar to clear. Yeah, it's very mobile, but that's also an expected thing at this point. Sure, it has a ton of damage deferrence, but any monster that can't break past that anyway is probably capable of exploiting the Berserker's more notable weakness to save-based effects.
In short, a lot of what gives martial adepts viability at high levels is how well-rounded they are. The Berserker, while plenty strong, rejects well-roundedness in the pursuit of smashing more face. At high levels, this falls apart. While it isn't a bad class by any means, even at high level (a barbarian would give an arm and a leg to do what this class is doing), it will likely fall short of the high standards set by the base Crusader.
As a dip
I mean, it's a Crusader variant, of course it's good for dipping, but I think this is a significantly better dip than the base Crusader. Having another strong class feature open from level 1 that keys off of initiator level doubles down on what makes initiators so dippable to begin with.
More specifically, this also makes the class remarkably good at blending with other initiators. Being able to trade a Warblade's or Swordsage's capstone for this Berserker Strength progression is a really potent option that greatly outclasses taking more levels in Berserker. This is especially true on the lower-health Swordsage.
The problem reminds me a lot of the issues with the Wizard class, actually. While Wizard is a ludicrously powerful class, nobody takes Wizard 20 because there are prestige classes that grant all of the spellcasting while also giving real class features. The same is true of this Berserker: the lack of strong class features will encourage players to leave early.
Addendum: Suggestions
I really like this variant. The idea of grafting the Barbarian onto the Crusader's frame is a good one, and Berserker Strength is an inspired way to do it. As mentioned, I think the class suffers from a lack of well-roundedness at mid and high levels, so since this variant class inspired me, here are some ideas of the kinds of abilities that could fill the gaps.
These are, of course, rough suggestions, and using all of them would be overkill. They simply give an idea of the kinds of things I think would do well on the class to bring it back up to the Crusader's level.
Feral Determination
The thrill of carnage grants you an unstoppable willpower. Whenever you drop a hostile creature, you gain a +1 morale bonus to all saving throws. This bonus increases by 1 for each creature you drop, but it cannot exceed one-half your Berserker level. This bonus goes away at the end of your turn if you have not dropped a creature since the end of your last turn.
Fear of Retribution
Beginning at 6th level, your fearsome demeanor can cause your opponents to second-guess their attacks at a crucial moment. Whenever you are the target of a single target spell or ability that forces you to make a saving throw, you may glare down the opponent targeting you as an immediate action. You roll an Intimidate check against the opponent as if perfomring the Demoralize action. On a success, the opponent's ability fails and the opponent is Shaken until the beginning of his next turn. On a failure, you automatically fail the saving throw for the spell or ability.
You may use this ability once per day. At 18th level, you may use this ability one addtional time per day.
Too Angry to Die
Your untoppable displays of rage have become so legendary that even death has learned to tread carefully in your presence. At 20th level, if you are affected by petrification or a death effect, it does not apply to you until the end of your next turn. If you would be cured of the delayed effect or made immune to it, such as by a Stone to Flesh spell for petrification or a Death Ward spell for a death effect, the effect is prevented.
Additionally, while such an effect is being delayed, you gain the benefits of Berserker Strength as though your hit points had been reduced.
Other class features
Any of Evasion, Uncanny Dodge, Track, Fast Movement, or Elude Touch (as a Wilder) would be good options.
