Try this- - - -Loop a cable come-along around the front of the engine and preload the assembly toward the rear as a diagnosis aid. Chances are part of the problem is that the torque tube is shoving the engine/trans assembly forward and changing the length of the clutch linkage. Once the judder begins, it will get progressively worse.
EDIT: How about rear spring bushings and/or shackles? If the rear axle assembly can move forward/aft a little, that can overcome the flex built into the motor mounts. The torque tube pivot ball could be rusted tight to the main tube and fail to slip like it's supposed to, as well. Just another thing to check on!
Jerry
