1983 300D Kerchunks Shifting

[handygeek]

Driving our son's project 300D the past two days I noticed that when it gets to about 40-50MPH & I back-off the fuel I hear/feel a kerchunk sound from the rear.

I notice it sometimes also when the vehicle is coming to a stop, but not as pronounced, and only occasionally.

Is this most likely ...

Universal
Rear Axle
Real Ball Joint
Transmission
Other

Need to sort if this is likely in an area we already paid to have repaired (rear axle replaced a year ago) or something new ...

Thanks - David