Armory currently has you listed at 577 Defense. That might be partly because your wep enchant, i am very unfamiliar with DK tanking but i can give you a small word of advice that will drop some of that def rating, swap out your shoulder enchant for the pvp +30stam +15Resil the resil is blah but the stam ups your health a tiny bit and only substitutes dodge. Also if im not mistaken you can swap the shoulder enchant 2chest gems, and bracer gem and still be def capped you would lose appox 9 defense.
4.93 rating =1 defense you would be losing 45 rating which is 9 Defense still keeping you above cap