Block wheels, tranny in neutral and climb under truck. Grab the rear driveshaft and give it a good tug, top, bottom and middle (by the slip yoke). If you feel any side-side or up-down play, that may be the source of the vibes. If eitehr flange, could be a loose flange nut, remove shaft, unstake nut and torque to spec., restake nut and reinstall shaft.