Can a Barbarian in Rage mode fight defensively or use Full Defense?
I didnt find any rule about it, I think there is something about using Combat Expertise, but nothing else.
In the session that the doubt come we end allowing the barbarian doing so, but seems odd.
In Dungeons and Dragons 3.5
Rage says, in part,
While raging, a barbarian cannot use any Charisma-, Dexterity-, or Intelligence-based skills (except for Balance, Escape Artist, Intimidate, and Ride), the Concentration skill, or any abilities that require patience or concentration, nor can he cast spells or activate magic items that require a command word, a spell trigger (such as a wand), or spell completion (such as a scroll) to function. He can use any feat he has except Combat Expertise, item creation feats, and metamagic feats. (PH 25).
That raging restricts other abilities requiring patience and concentration, but never specifies what those might be, means the DM has a free hand in declaring certain tasks require patience and concentration and therefore are beyond the raging barbarian. But the DM should probably rarely place such restrictions on tasks, given the limits present later in the description of rage.
Thus, as no mention's made of the raging creature being unable to do it, fighting defensively (PH 140) remains an option, as does total defense (PH 142).
It is odd, but the Player's Handbook just doesn't restrict rage beyond these limits. The game says that if it's not on this list (or specifically prohibited by an option in a later book), the barbarian can probably do it while raging; so it's perfectly acceptable for a barbarian, for example, to apply extra sneak attack damage if he has any, the target's vulnerable to it, and the circumstances mean he'd inflict it; fight defensively or use total defense; attempt disarms or otherwise use nonlethal force; and take the withdraw action.
The goal is to limit the raging barbarians' actions in exchange for increased combat prowess not precisely simulate violent psychopathy in a table top role-playing game. In this case an exclusive list is better than an inclusive list (i.e. the raging barbarian can't do these things versus the raging barbarian can only do these things) because the game knew dozens of books with far more options would follow, so rather than limiting the raging barbarian to actions--requiring every future option to note if a barbarian could use it while raging--, it prevents him from taking some actions, so the raging barbarian can use future options freely.
In Pathfinder
Rage says, in part,
While in rage, a barbarian cannot use any Charisma-, Dexterity-, or Intelligence-based skills (except Acrobatics, Fly, Intimidate, and Ride) or any ability that requires patience or concentration.
And then omits the remainder. Without the rest of the list it's in the DM's hands as to what, exactly, requires "patience and concentration."
A cursory search of the skills turned up nothing explicitly requiring patience, in feats only the aptly named Patient Strike would seem to need it, and the bonus from a traveler's dictionary is increased if the user is patient. But the concept of concentration is loaded with meaning. Concentration is what's needed to cast spells ("To cast a spell you must concentrate"), for instance, and at least one use of the skill Use Magic Device requires concentration.
But the exclusions given for D&D 3.5's rage aren't otherwise present, so a raging barbarian can use Combat Expertise, magic items, and other D&D 3.5 restricted options if the DM rules these don't require patience or concentration. Thus it is up to the DM whether a Pathfinder raging barbarian can use the total defense action or fight defensively.
There's nothing explicit in the rules about it. As @HeyICanChan points out, there's a long list of things that a raging barbarian can't do, and fighting defensively (or even using total defense) isn't on that list.
At the same time, the list of restrictions isn't exhaustive. One of the items on the list is "any abilities that require patience or concentration." It's a plausible thing to say that fighting defensively requires patience and concentration, and is thus restricted. Certainly you can argue from realism: if you're not carefully keeping track of incoming attacks and watching for openings to attack yourself, then you're not going to be very good at it.
But that is a ruling, not RAW. If you're comfortable with that, then that's fine: the reasoning is easy to follow. Just be aware that some DMs may see things differently.
