Barbarians are a bit easier to damage than they should be (or at least I think so). So, to fix this, just give them a 10 to 20% reduction in physical damage at level 20, or spread out the bonus over the 20 barbarian class levels. This reduction in incoming physical damage would work a lot like PRR, but it's not PRR. Similar to how one can have both a dodge chance and an incorporeal miss chance, this reduction in damage would be calculated separately from PRR.
I feel that barbarians need this PDR (percent damage reduction) since the class isn't a naturally self-healing one. Since they have poor AC, they should be better at mitigating that damage they inevitably take in combat than other melee classes.