Sorry, Do you meen the engine & box will have to come out or are you saying
it can be done in situ?

Keith, are you trying to frighten him?

It also sounds exactly like a driveshaft has come disengaged from the
splines on the outer CV joint or the CV joint has failed. There are
quite a few failures which are much less dramatic and cheaper to fix
than diff problems which can cause this symptom!

Yes, Graham is right, it could be that, but the fact that the speedo works
tells you that the gears are engaging and driving the gearbox 3rd motion
shaft, but the drive is not reaching the wheels. With a driver in the car,
engage a gear and then carefully look under the car to see if either of the
drive shafts are turning. If not, then it is the final drive gear as I said
in my first reply. If one of them is turning, then it will be as Graham
says. Also I have come across a problem where the spline on the end of the
drive shaft at the wheel end has worn away and the shaft turns in the wheel
hub without turning the wheel. Un fortunately if it is that big nut then the
engine has to come out. Normally we split the gearbox from the engine for
this job, but I think it may be possible to do the job through the gearbox
end cover. The difficult part will be getting the nut retightened to the
correct torque and bending up the tab washer under the nut.

Good luck,

Keith