I was reading the rage powers of the barbarian and fearless rage says you are immune to shaken and frightened. Since shaken < frightened < panicked, if you cant be shaken does that prevent the worse conditions as well?
No, being immune to Shaken gives you the wherewithal to ignore things that would normally be intimidating or downright scary to you, but not the ability to shrug of things that are truly horrifying.
You would not be vulnerable to enemies stacking various Fear effects on you, but if something makes you outright Panicked, you still Panic as per the condition.
The ability doesn't say you have immunity to Panicked, so you don't.
For reference, I don't consider greater Conditions to be "secondary effects" of lesser Conditions.
Immunity (Ex or Su)
A creature with immunities takes no damage from listed sources. Immunities can also apply to afflictions, conditions, spells (based on school, level, or save type), and other effects. A creature that is immune does not suffer from these effects, or any secondary effects that are triggered due to an immune effect.