Get some paint and mark the axle and propshaft flanges. Remove bolts, rotate the propshaft to a different position and bolt up.
Then try again. Might not cure or point to anything, but it is the cheapest answer. Old books always told you to mark the propshaft on removal to stop vibrations from a mis-placed shaft.
I think that this advice would refer to the propshaft itself, if you were to separate it at the spline. The universal joints at each end must line up correctly to cancel out the speed variations caused by them running at an angle. I can't see that changing the position on the axle input flange would make any difference.
Anyway, the problem seems to be engine speed related, not road or propshaft speed, as it occurs in all gears in a particular, narrow, rev range. Something vibrating in sympathy somewhere, but I cannot suggest what it might be. Maybe something attached to the engine - exhaust pipe, dynamo mounting, fan belt????????? Or inside the engine - oil pick-ups, baffles???????????
