453 defense??? thats where your problem lies. HP is a bit low too but your defense should be way higher than 453. My DK has a 470 defense and I'm only level 78. You should aim for 540 and the closer you can get to that # the easier heroics will be. At 540 you will no longer get crit, but also as you climb towards it your parry and dodge % will go up.

Best bet is to use the wyrmcrest and argent crusade tabards to get reverred with both factions and get the tank pieces from them, that should help a lot.