pistachioted,
Are both rear motor mounts missing? Your mechanic must mean they are oil soaked, broken and need attention, but not 'missing'!
Whatever the cause, it appears that it doesn't happen until the truck is under a load. It could be a bad u-joint, and screwed up rear motor mounts would exacerbate this condition for sure!
I guess the bottom line is, your mechanic indicates you need rear motor mounts, so put them in and try another load of hay! if you get the same vibration, a u-joint might be the next step.
If the engine is in poor shape tune-up wise, the strain the load of hay put on it may have caused it to miss under load, which would shake things up, especially with bum rear mounts.
You have lots of things going on and more research is required to pin down the problem. Good luck.
Stuart
