http://www.wowarmory.com/character-s...&cn=Miahalenge Is this you?
If so the TPS issue is pretty simple.
http://www.wowhead.com/item=51010
Heroic versions of Bonebreaker or Facelifter 10m, LK 10 man tank wep (N or H) would all improve your threat.
Or, something that would really help,
http://www.wowhead.com/item=50654 off you're next H Lootship that it drops. Yes, it's not a tanking wep per say, but it is a EH upgrade, you don't need the Defense, and it's a 25% wep damage increase.
You've mentioned the DK that is way above you in threat, does he happen to be using Shadow's Edge, Bryntroll's, or even Shadowmourne (because you haven't linked)?
My guess is there's a very big difference in the output of your weapon and that's where the threat difference is coming from.