I hope I'm not repeating a question that's already been asked, but here goes:

One of my axle shafts on my 1981 300TD has an obvious problem. Not only is the inner boot split but, more worryingly, the shaft can easily be moved by hand by about an inch in and out along the direction of its length. Driving forwards causes no strange noises (yet) but in reverse the shaft moves in and out as I described and makes a nasty knocking sound. The other shaft is OK - boot still intact and no movement. I suspect the movement came first and that's what caused the boot to split.

So what can be the explanation? I don't have a very good idea of what these parts look like inside, but my suspicion from reading this and other forums is that the C-clip inside the diff has broken. Is that possible or likely, or can anyone suggest any other place where it might have come loose?

I'd really like to know what to expect before I start the job so I know whether I need to replace just the boot, or the shaft, or both shafts, or pick up a used one from a wrecker. These parts are very expensive where I live (in Australia) so if I buy any new parts I'll probably be ordering them online from the US and waiting about 2 weeks for delivery. Which makes getting the order right first time all the more important.

Many thanks in advance.