winmutt: There is no rubber shock mount for the differential. It bolts right to the frame without any shock isolation. It is isolated from the driveshaft through the rubber flex disk, and from the wheels through the CV joints in the axle.

dropnosky: The problem happens to some extent in all gears. It is not very noticeable in 4th and 5th gears because the engine cannot apply much force in these gears. It is not usually a problem in 1st gear, since most of the time the driver is accelerating fairly hard in this gear. To cause this problem in any gear, all the driver has to do is make an abrupt throttle change. At this point, I agree with you that it is probably not the drive train causing this.

I am starting to think this is a throttle linkage problem, or perhaps some strange inertial interaction in the injection pump or governor, but I don't see where it could be. All the linkages seem tight to me. It was at one time an automatic, and there was a linkage from the bottom of the injection pump down to the auto transmission, which I have removed.

I once had a similar problem in a 1965 Buick Special, and I discovered that one of the motor mounts was completely broken. I could stand in front of the car with the hood open and the engine running, and by "goosing" the throttle linkage with my hand, could make the V6 engine rise several inches out of its mounts. Because the throttle linkage was routed in from the side, this motion would reduce the throttle which would cause the engine to settle back down, which would increase the throttle, etc.... I don't think this exact thing is happening, since the motor mounts are new, and the throttle linkage, from the engine to the firewall, is a cable, not a rod.

Stevo, I just replaced the clutch. I am thinking how to make a video of this behavior. Maybe in the next couple days.