Heheheh, all good now ...
It cost a new Transfer box flange, an X-Eng disk conversion and general fettling/replacing/lubricating .. Oh, and new hubs, bearings etc etc ..
Nothing to do with the rear diff, the output flange was cracked and I reckon in use was opening up and slipping splines, thus loosening 'everything' ..
The prop/flange nuts were also not tightening properly, when I took the flange off again to check stuff it took a while to notice, but the heads had been welded onto the flange, with no circlip, and had broken off again. So at the same time I've replaced the flange, the bearing, seal, and all other replaceable bits in that area. Then, to make sure, I also stripped the rear prop down to find no grease at all in the splines lower down, but fine by the nipple. So cleaned the crusty mud out, toothbrush helped, but tasted awful afterwards. Re-greased it at the bottom and along the splines and it now expands and contracts as it should, which has to help! The rear diff is as nice as it should be so did nothing to it except replace, as usual, the EP90, 'cos I've also replaced the rear hubs, calipers, wheel bearings etc, effectively an almost full rear drivetrain refurb. No clunking (yet) but it ain't been laning either ..
X-eng disc conversion just works. Second hand, can't afford new, so cleaned up, greased the spring, new pads etc and job's another good 'un!
Just need to re-bleed the brakes now 'cos although they bled up and felt great when I finished the axle bits, someone left the master cylinder cap off and on the first test drive, we lost a bit of fluid! (It wasn't me!) so the first press is a mite spongy!