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
I'm not sure I would recommend this. The NHRA viewed this type of activity so unsafe that they no longer allow vehicles in the pits to use jackstands in this way. You have to use a stand that you can pin to the chassis if you want to run the car in gear. A way to ensure the vehicle will not come off the stands. When you see them "whack" the throttle during tune ups, those vehicles jump around so much that this makes a lot of sense. I definitely would not recommend getting under a vehicle that is running and in gear! That's just asking for trouble.
