Admittedly, I've not raided so this might be incredibly dumb and I don't know, but at level 10 marauder gets a move called Brutal Swing which is an instant cast stun that's not on the GCD with a 30 second recast, so why not take a Warrior for it? If you want a PLD tank just make sure the warrior's not using Defiance, and they should be able to stun-bot easily and provide a little extra damage.