Vibration comes and goes at 50 to 70mph. Feels like it in the rear.

 

any lift..ujoints?

 

No lift and u joints are new. Bearings in the rear diff?

 

Is it a vibration or a growling type noise?
Rear diff bearings and/or gears usually make noise first, same goes for wheel bearings. I would say if they are causing a vibration, they would probably make noise too. Does the vibration change under accel or during coasting or when turning? Did you change the u-joints? If so did you see any damage to the yokes or driveshaft tube? Get under the Jeep and twist the rear driveshaft, or try to rock it side to side and check for play. Go ahead and check the front too since you're down there

 

Have had the tires rebalanced? Start there.