Something is out of balance on the driveshaft. Did it come with the Jeep but it wasn't installed? If so, then that's a good sign the previous owner removed it because it had a problem. It could be bent, or a u-joint could be bad. I'd take it to a driveshaft shop and have them look it over. I don't mean a garage or 4x4 shop, I mean a driveshaft shop. Those are the guys that can figure out what's wrong and not rape you on the cost as a garage shop that would just send it to the driveshaft shop for repair anyway.