2003 3500 Dually, 6spd Manual

Recently, I asked about clutch/driveline shudder on clutch take-up (when the clutch is almost fully engaged).

I ended up:

1) Checking driveline angle
2) Replaced Clutch Assembly (OEM type organic lining), PP, throwout, etc.
3) Resurfaced Flywheel
4) Replaced all driveline U-Joints and Center Carrier Bearing
5) Replaced Rear Trans Mount

The shudder is 95% GONE in forward gear, but when BACKING on a slight incline with even a light trailer, the shudder is INTENSE!

The front of the entire truck starts hopping up and down that not only does it rattle the teeth of the driver, but observers can see the front end bouncing up/down at a very rhythmic oscillation rate.

There's no feedback (vibration) through he clutch pedal, or in the gearshift lever.

The movement appears to be straight up and own at the front of the truck, and you can't much rearward force to reverse the truck.

This is at idle rpm (foot OFF the throttle completely), or elevated rpm feathering the clutch.

The engine mounts LOOK good, and there's no noise from them, but I will replace them in the next few days. There's nothing else to change after that!

Any NEW ideas?