|Have you tried this? Get both rear tires off the floor, put car in gear to lock DS. Gently rock one tire a little CW and CCW to feel slack in 1/2 shaft joints. Now isolate outer joint and gently rock tire to see if inner 1/2 shaft joint at Diffy has rock slack, maybe inner and outer joints have rock slack and that is the clunk issue if slack excessive?
I recall now that I did this some time ago and did find slack at inner and outer joints but I did not know the spec for the slack so I just assumed ok.
Since you tried different diffy's and still had clunk so probability not diffy internals.
Have you ever replaced replaced the 1/2 shafts or rebuilt those joints?