defence rating is to reduce crits to 0% (490 skill to be exact) for PVE
defenc rating also give + to Dodge, Parry, Block, and Miss
Resilience reduces crit for both PVP and PVE but is kind of a waste in PVE
Resilience also reduces crit damage, wasted in PVE if you cant be crit
Resilience also reduces DoT damage, dont know if that works in PVE or not, but still pretty much a waste. as there are no affliction warlocks for bosses (to the best of my knowledge)
you need to reduce crit to 0%
you need an avoidence total (including block % even if by definition is not really "avoidence") to equal 102.4% to prevent crushing blow.
if those to things are not met. it wont matter how much HP your tank has.
Resilience CAN replace defence rating, i think. however. Defence rating also adds to avoidance numbers.
now its my opinion that a warrior tank could possible tank with MOSTLY pvp gear, not all, as enough res can push off crit, and shield wall can get you uncrushable. but you will still lack other avoidance values like dodge and parry. also note that extra avoidance is needed for War tanks as if any boss attacks faster then 2.5 sec(shield wall's limitation), warriors will be crushable for bit, along with any parries the boss gets will also mess up warriors crushablility. so. with that said it should be possible.
a pally in pvp tank gear cannot ever tank end game in PvP gear. due to them needing every single peice of gear they can find having a very good amount of avoidance on it. due to war's shield wall having +75% block, and pally's holy shield having only 35% block (if after the right libram is aquired).
and i have no idea how druids work to give a good opinion about them. but they do get -3% crit talent, making them need far less Defence rating. and to the best of my knowledge, they cant reach uncrushable anyway so they rely on massive HP to make up for that. to me it sounds like you need crazy Healer Mp5 to make a druid tank work out. but again i dont know squat about druids or healers.
Edited, Dec 7th 2007 2:18pm by RuenBahamut
EDIT: See'ing as the only true limitation to this question is the tank vs PVP/PVE gear. maybe this question would be better answered in a War or Druid forums. someone may have already done it in all pvp gear.
as a prot pally main, i can tell you that prot pally will need a miracle if he can survive in only PVP gear. or even half PVP gear for that matter.
Edited, Dec 7th 2007 2:34pm by RuenBahamut