Steve, you don't mention what half shafts you are running. If you are using the halfshafts that use an inner CV's on both ends, I would bet that you are not using an end stop and have pulled one of the CV's out of the housing.
Put the rear of the car up and feel around the boot on the CV's. One will feel different that the rest. Pull that shaft apart and inspect the inner lip of the CV housing. It will be deformed. THe opposite end will have an end cap that is deformed.
If you are not using these shafts, check that stub axels.
Let us know what you found