A few weeks ago I put a 3 inch lift on. Shortly after some grease came out of the rear DS where it meets the tcase. I put in some rear arms after to adjust caster but I am not sure if DS is damaged. When it is cold I hear a like rattle hum while the jeep is under load until it warms up. I can't see it but is it possible I tore the boot and the DS is going?

 

Caster has to do with the front axle. If your jeep is a 2 door and you put a 3" lift on which may have netted you more lift than 3" it is highly likely you have damaged the rear driveshaft. Only because of the severe angle I'm sure it is at. Post up some pics of the problem area. I'd recommend getting a cv style driveshaft with replacement yokes not flanges and control arms to adjust your pinion angle

 

I put the arms on but a week after the lift. I plan on Ds but don't want to guess at the problem and throw money at it. I'll try to get a pic tom.

 

Pull the rear shaft, throw it in 4wd and go for a drive. As long as you don't have a front auto-locker, should be an easy way to verify rear shaft noise.