No U-joint back there, there is only one in the center.

Check the flex disk on both ends of the driveshaft, chances are one is bad (a thump on downshift is a common symptom).

Do not delay, if it's bad enough to thump, it will fall apart sooner or later, and the driveshaft can do some serious damage if it comes out!

While down there, check how far you can turn the driveshaft before the wheels turn (car jacked up). If more than half a turn, with a clunk from the diff, the dif spider head is worn out and you need to replace the diff. Flex disks are more likely.

You should also inspect the differential mounts, rear subframe mounts, rear control arms, and rear control arm bushings. If the bushings are bad, you will be able to move the rear wheels back and forth with them off the ground. you will also have noticable torque steer.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!