My 1982 300D has a loose / worn differential.

Pretty easy to diagnose. Park the car someplace where you can get under it far enough to turn the driveshaft by hand. Set the parking brake (use wheel chocks if necessary) and put the transmission in neutral.

Now reach under the car and rotate the driveshaft back and forth. If you can turn the driveshaft back and forth while the wheels are standing still, your differential is "loose". And while under the car you can make sure there is no slack in the half-axle shafts, because they probably won't be turning either.

I get about 60 degrees of a turn looseness. That's pretty bad I think, and it does the "clunk" in transition from acceleration to engine deceleration.

Ken 300D