Hi Xerxes . . .
The oscillation on the pinion end of pumpkin suggests a slight axle wrap twisting the pinion end up against the leafs and then rebounding off the springs. Stroker may be on to something in terms of drag from the brakes causing the axle to start to wrap in the first place.
I haven't got my head around why the larger tires would magnify this effect. But I am also puzzling about the impact of running the drive line while up on jack stands without any weight on the wheels.
You mention that it only happens at low rpm. So it smooths out as RPM increases?
regards,
stock49
