A commonly overlooked source of "clunk" in the rear of a 124 car is the differential mounts. Unlike earlier cars, the differential is mounted in rubber mounts.

One mount is at the front pinion part of the diff and has a bolt that goes vertically through a couple of large rubber "washers." Those will usually not give a clunk.

The other two are horizontal with bolts that go from rear toward the front at the top rear of the differential mounting the diff to the subframe. This is a somewhat common failure.

To check them, raise the car safely and look at these rubber bushing looking mounts at the rear of the diff. If they have failed, the bolt will not be in the center of the bushing and if you raise the diff slightly with a floor jack, or MAYBE by hand, you will see those bolts move in the mount.

Good luck,