If you have the time i'd try and work your DK's and Shaman teams together towards 80.
It may seem alot but bouncing between the 2 will give you the benefit of rested xp in making the trip to 80.
Plus you will have the advantage of swapping out for a tank for instances or tough fights.
DK's are great for levelling but for me Shamans are a must at for 80 IMHO
Hope this helps