http://www.wowarmory.com/character-s...nd&n=Chernabog
I don't even get the chance to use Pestilence to spread diseases until I hit LBRS. The key is keeping up Unholy Blight so you tag each mob and the pet will do a lot of the work. Once you hit LBRS you can round up a bunch and give them one disease, then hit a fresh mob and spread the other one. One disease is enough to kill any of the mobs there, but it does go faster if you can get both going.
A 58 DK would be able to do up through SM quite quickly. That's enough to boost to level 44 or so and would be much faster than hand grinding.
Edit: I think the only video I could do that'd be helpful would be LBRS to be honest. The rest of them are self explanatory. Is that something you'd want to see?