Stick Shift Puzzle

I have a Grand Cherokee (1994) with 70K on it. It's one of the last
stick shift Grand Cherokees made, and I've owned it since it was new.
I've developed a shimmy in the drive train that feels like the engine
and transmission want to fall out of the vehicle. It's so bad that
I've had to pull over to the side of the highway in stop-and-go uphill
situations and wait for the traffic to clear. Before you suggest the
obvious, here's what I've done: new clutch (including pressure plate
and flywheel), new shocks all around, had the motor and trans mounts
checked - twice, plus the universals, and the brakes are new. The
brakes are not the problem because the vehicle rolls easily on even a
slight grade. Here's the last bit of the puzzle: when cold, it acts
perfectly OK: no shake at all. When I've been on the highway for a few
miles and then try to start up a hill (even a small one, like my
driveway), then the problem shows up. The car doesn't want to move and
the whole underside of the motor/trans shakes like crazy. When I'm
having this problem, I can hold the car with the emergency brake and
try the clutch - no shake at all. I've taken it to various shops and
no one can figure it out. Any ideas out there??