Either give PLD some form of -damage taken ability, or simply adjust the way defense works against damage. Right now, stacking defense has very limited utility after a certain level, when you get right down to it. Adjust it so that having a substantial defense stat (and probably giving PLD more Def Bonus traits) makes it so that the new monsters of endgame don't utterly destroy you, and you'd give PLD a reason to be played again.
It's possible, but you'd have to do some fairly serious work to make it worth taking PLD on a regular basis again unless you give PLD the ability to gain and hold hate in excess of any other job in the game.