pistachioted,
So the vibration is still there..you can get it to do it when you want..right. If the engine were missing and flopping because of bad mounts, you would surely see the gear shifter flopping too.
A bad u-joint might fail and cause a vibration and this would stay. You would probably feel this sort of thing in the seat of your pants. Could the load have caused a tire to separate and start thumping. With the rig lifted off the ground you wouldn't notice this type of problem. Run your hand around the circumference of each tire and hunt for a lump or bump.
The frequency of a bad tire and the frequency of a bad u-joint would be quite different. See if you can recreate the problem going down the road empty then try to refine your troubleshooting to narrow it down.
Stuart
