I would really like to add that when you reach around 80% Dodge + Block + Parry + Miss, Defense raises avoidance more per point than dodge or parry.
102.4% dodge + block + parry outside of ICC is the 'avoidance cap.' This is the point at which you can no longer be struck by anything but Glancing, Miss, Dodge, Block, or Parry. In heroics, this means you pretty much hardly need a healer, if at all (My pally is capable of 1 tank, 4DPS in the easier heroics).
Personally, I would go for a Nighttime axe off of Brownjames and keep the rest of your gear as tanking gear (though, the Ret 245 libram has been tempting me for a while).