I was easily able too surpass the 540, the required def too tank, but now as I gear up in naxx, I've dropped back down too 540, when I was easily 560 before. Could someone tell me a few good upgrades def wise I could grab so I can continue too gear up without worrying about being under def cap?
It looks like you don't have a shoulder enchant - get the hodir one or your exalted scryer/aldor should inscription and you'll pick up a little more def rating.
Also, you could drop a stalwart or glimmering monarch topaz in your shoulders and pick up defense from the gem and the socket bonus, and/or do the same with the red socket in your pants tog et the +6 def socket bonus there. Obviously you don't want to sacrifice any more stamina than you need to, so pick socket bonuses that will synergize with your needs.
I had the same problem as you when I was gearing up in Naxx. I would get an upgrade and it would put me just below the cap at like 539 or so and was annoying. Use the BC flasks if you have to (500 hp 10def) which are pretty much point for point just as good as the Stoneblood flasks or whatever they're called. Also I had to switch a gem or two around also whenever I got anew piece to stay above the cap early on.
Carry a +8 def/12 stam gem and a 16 def. gem with you in your bags, so when you get a new piece of gear, you can socket it right away which might help you stay above the cap.