Am I wrong to ask the tank to get 490 def b4 anything else?

This ^ is my problem. I have spent so long stacking stamina that now I'm having a problem getting uncushable and every time I 'upgrade' to gear with better avoidance I lose stamina. I really think I'm going to just bite the bullet and drop all my stam gems for avoidance till I have my avoidance and deal with it from there.

As a tank I definately have seen the difference with getting closer to being uncrushable in how much damage I take. I'm not comfortable with heroics yet but I'm having to start tanking topless to take enough damage in normals so that I don't run oom. It's a nice feeling.
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Honestly, one of the easiest ways to deal with this is by changing your enchants, not your gems. Getting defense to bracers, defense to chest (especially chest given how crappy the enchants are there), and most importantly shield block rating to the shield can make a huge difference - much more than gems. And it's a lot cheaper most of the time.