You have 530 defense currently. Don't ever try to tank any boss in Naxx 10 until you are uncritable (540 defense).
Health is also too low for Naxx. I'd recommend 24k+ unbuffed.
Some DK tanks like to dual-wield. Most experienced DK tanks tank with 2-hand weapon. First, if you DW, you attack more frequently, which means boss is going to parry more often. When boss parries, its next attack is faster. i.e. you are taking more damage when DW. Second, Blizz added the Rune of the Stoneskin Gargoyle http://www.wowhead.com/?spell=62158
in 3.0.8, which is a 2-hand weapon only enchant. It adds 25 defense, which is nearly ~100 defense rating.
Check this site out for more information about DK: http://deathknight.info/forum