I'm not sure about ToC, but Ulduar should be possible - some fights you can maintank, and you should be able to handle offtank duty quite well.
(Good knowledge of the fight helps a lot.)
As it stands, I'm gonna say this about defense: The more the better.
I get almost as much pure avoidance from defense as I get from dodge rating, (within a few percent, in fact,) and I consider the avoidance from defense to be quite a bit better then that from dodge. Part of the defense avoidance is parry, which has it's advantages, and part of it is miss, which is the best avoidance there is since it works from the rear and while stunned (I think) or casting.
So, the more defense the better, and never ever go under 5.6% critical strike chance reduction.