Do you have a SYE or transfer case drop? How big is the lift? My guess is that the junkyard shaft is out of balance. The rear driveshaft would be the shaft that is most affected because of a lift, simply because it is so much shorter, and changing the height has a huge change of driveshaft angle. If the rear driveshaft has no vibrations, the front should have no problems at all unless the pinion angle is totally wrong in the front (ie the previous owner got aftermarket control arms) Have you checked to make sure the front and rear gears are the same? Post some pictures of front and back axles and their angles.