It depends on what level characters you have available. Here's what I've been doing...

I leveled my DK with the starter quests up to 58, then have been running him through Mana Tombs with my 4xShaman team (who is level 71-72ish). Mana Tombs is super fast and the mobs are still green to the low 70s shammies, so the XP for the DK is pretty darn decent. With full rested, it took me about an hour/level to level the DK from 58-62. Every week or so, I grab the DK a level, then put him back at the inn to accumulate more rest XP.

This won't work if your team is L80, but if you have an alt team of low 70s, you can probably farm BC instances with them and help powerlevel the DK along. Bonus: my shammies slowly level as well from the runs...