I recently leveled a DK and, like you, realized that queing as a tank made life simple. Here is what I did: I chose to head to reg TOC and get as many upgrades as possible, both tank and dps gear. I then focused on gemming and enchanting my new dps gear and start queing for random heroics as dps. On my server the que time as Dps is around 15 mins. I don't know about you guys but before this cross-server lfg function became available it would take at least 15 minutes to find a group.
Next thing I would suggest is to take any upgrades you can and spend your emblems on your T9 tanking set.
By the time I was done accumulating emblems and upgrades I had full T9( both dps and tank set) a ton of cash and a lot of confidence in my ability.
The only downside is that this route requires some patience but I managed to take my DK from a new 80 to a toon that can both tank or dps equally well.
Good luck to you mate!
Chestypuller 80 DK|Mechanism 80 Warrior|Popejonpaul 80 Priest