Grind in Outlands to 60.
The xp per mob is double. You divide it four ways, then get a group bonus.
You get about 75% of what you'd get for a solo kill in the old world, but you have multiple times the firepower.
I did this solo with a mage at 57, getting 500+ a kill, 1000+ rested. You should get at least half that much and be able to mow things down. You can also start quests at 58. Don't worry about running out of quests in outland either, there are just tons of them.
