Stephen, are you running the approved
oil for the powr-lok diff?.
It may not help if the diff is stuffed but the PL diff uses either:
Castrol hypoy L.S.
BP limslip 90/1
Shell S8096B.
These are straight out of the Jag manual for PL diffs but they may be superceded by later, newer, and better diff lubes.
Cheers