I'm a fan of high stam. Especially for Uldaur -- you want to have a solid cushion of HP for your healers to react. The only reason to consider stacking +def is to build more avoidance, but you're usually better off stacking pure dodge/parry/block. I've not seen any successful def stacking tanks, but that's not to say they don't exist.
21k hp for starting Naxx? Maybe in optimal conditions with an amazing healer and a solid MT you could get by, but it wouldn't be a breeze.
With your current gear you could probably start 10 man naxx, while you're in there, talk to your healers and see what they have to say.
Just say: "How am I doing?"
You'll get one of 4 responses:
1) "You're dropping in big chunks." -means you need more hp.
2) "You're consistently losing more than I can heal." -means you need more avoidance.
3) "You're not holding aggro well enough" -means you need more hit/exp.
4) "You're doing fine." -means keep doing what you're doing.
>>>Hallelujah<<< Lightning's Blade Maintankadin
Unbuffed: 30934 HP, 105.66 Avoidance, Crit-immune