Both of my P38s have made a click/clunk/clack noise shortly after you start moving when reversing. Now though the noise occurs when you pull away, more often than not. More than anything its just annoying.
The LR TSB outlines resolving the problem by applying Loctite 648 to the splines in the diff end.
I left the car with the local indy to sort a few bits and see if they could find the cause, if not, look at the TSB and perhaps do that. They weren't too inclined and raised an interesting point... if the half shafts are loctited to into the splines, how do you remove the hub/axle to replace a wheel bearing?