vibration in 300E

I've got a an '86 300E with a vibration/rumble sound that occurs at speeds above 65 mph, and rapidly worsens with increasing speed. It seems to be coming from the rear of the car, and can be felt through the seats and 'in your chest'.
I've replaced the following so far:
- motor mounts
- transmission mount
- front and rear flex disks
- had drive shaft balanced with new center bearing and u-joint (vibration dampener was removed at this time)
- changed wheels and tires
- changed differential and transmission oil
The previous owner has replaced the rear differential mounts and rear subframe mounts, and one rear wheel bearing about 20k miles ago.
So far nothing as made a difference. The car has 220,000 miles on it, and has the 5 speed manual transmission.
The rear differential whines at low speeds, and there is a clunk from the rear end if I don't clutch smoothly.
What's the next most likely culprit?