My 79 300D also makes a clunking sound when rocking it forward and backward on flat ground. The sound comes from too much play between the spider gear shaft (in the diff) and the drilled hole that the shaft fits into. I would say about 1/64" play on both ends.
The clunk only occurs when there is NO load on the drive train while upshifting, particularly between 2nd and 3rd at about 40 kph (25mph). As long as there is a load (accelerating even slightly) the clunk is decreased or eliminated. I decreased the noise level of the clunking sound somewhat by switching to 85W-140 gear oil.
Some history on the car: The PO was a complete dingbat that didn't maintain the car. When I bought it from him, the tranny shifted from 2nd to 3rd with a "bang" at about 50kph (30mph). It would not shift until that speed threshold was reached, and then it would "bang" into the next gear. Only a dingbat would put up with this for any length of time without getting it fixed. Could be, years and years of this caused the spider gear shafts to deform
I have since adjusted the shift points and added some tranny conditioner, so now the tranny shifts OK and the clunk is not getting any worse. The clunk can be minimized by keeping a load on the drivetrain at the shift points. The diesel is only 75 hp, so I think the problem is gone!
Why did I even buy a car shifting with a "bang"? I got it for a good price and I figured I could fix the problem.