If your Wrangler is lifted, you might have trouble with the CV joint at the front of the drive shaft where it connects to the transfer case. I suggest as the others above that you first check your tires, but you say that the vibration seems to be in the rear, and only happens when you are decelerating or coasting, which suggests wear somewhere in the drive shaft, drive shaft connections, rear differential, axles, etc. It sounds like wear that offers some degree of freedom of motion when there is no power being transferred from the transfer case. This assumes that you can't feel the vibration in the steering wheel. I would suggest setting parking brake, chocking the wheels, putting the transmission in neutral and crawling under and checking for slop up and down and side to side in the drive shaft linkages. After that, make sure the front wheels can't roll, jack the rear end up and place jack stands for safety. Release the parking brake, then trying wiggling the wheels top and bottom and front and back. There should be no slop. They should spin but not wobble. Also spin them and watch and listen closely for any rumbling or grinding. Inspect all the boots over the various linkages and connections for signs of leakage.