thanks Chris. I have used threadlock in the past, to no avail. The studs seem to shear not through rotation but through intermittent tension I think. The diff is in good condition. I rebuilt it approx 50,000km ago but previous to that (150k km ago?) a bit of loosening off occurred that I managed to get on top of. And previously on a manual VX, the same happened. In that case I didnt personally do any of the repairs but a local mechanic did.
My theory is that the axle tube actually flexes causing it to "hinge" at the housing area, thus pulling away from the diff. The studs usually tend to start failing at the top. Could weld a web across the top and/or front of the housing to stiffen it up perhaps.
or just weld the diff on - unlikely to have to rebuild or dismantle again.
I have had the vehicle for 9 years now and mileage is 365000km plus however much they would it back before I got it.