Put the rear axle on jack stands, run the engine at idle & in gear. Crawl under the truck with a piece of chalk & hold it close to the driveshaft in the center & towards the ends. The marks on the shaft will show any runout or bent shaft. Check the driveshaft for any dents. Sometimes guys will put the shaft tube in a vise while replacing U-joints & put a dimple in the tube. Those can cause vibrations.
George
