Inner CV / Differential Problem

[Pikester]

Hello chaps,
I'm helping a friend out with his MK1 1.6 MR2 (1988) -

The shaft that goes into the diff (between the inner CV and the diff) is wobbling around quite badly, to the point that the oil seal on the gearbox casing is starting to die.

I suspect that its probably the bearing on the diff thats to blame - is there any other likely cause for this problem?
I was thinking maybe the splines aren't engaged all the way allowing for some free play or something like that.

Assuming worst case senario, how hard it the differential to service? Is it a gearbox out job or can it be done in the car?

thanks chaps,

Mike