I forget in which thread the strategy was mentioned, but you can grind the bottom floor of Underbog (up to the first boss). Reset and repeat. You'll probably gain a lot of xp from 60-65 and it might be faster than boosting through ramps. I boosted my team from 65-68 using that technique. I got about a level every 40-50 mins. You may hit the 5 instance per hour cap doing it, but it worked pretty well for me.