Cheers for all the suggestions folks!

My engine alignment & end-float are good...I always pay particular attention to that. Something I hadn't considered was the upgraded bracing for the centre diff. Certainly worth looking into.

I have been using rubber sealed bearings though. It was all I could get at the time. I've ordered some metal shielded hybrid ceramics, just to try them.

I'm replacing the spur, bell, shoes, bearings and shims as soon as the new bits turn up.

Again, thanks to all for taking the time to respond