Without being there only so much troubleshooting can be done via the net
It may not be the slip yoke, however I would suggest having the slip yoke lubricated and go from there, it certainly won't hurt anything to have it done and it also gives you an opportunity to inspect and or replace the U-joints while your there... if the symptoms are still present afterwards you will know what its not... I know that sounds rather shootgun-ish but if you have had it in the shop and they have not be able to find anything its probably the best route to try....
Consider some other items to check:
- pull off the rear wheels and inspect the rear brakes for proper fit and ware of pads and calipers
- Check the rear shocks mounting grommets and bolts
- Check all exhaust mounts
- Check exhaust for rub marks or other contact marks to the frame or body
- Check the rear leaf springs and shackles for wear and fit