Agree with Rob, If engine/pinion alignment was so out of alignment then you'd feel it way before you flogged the diff to bits. Remember, ideally, the centerline of the engine and the centerline of the diff pinion should be parellel in both planes or at worse within 1/2 degree of each other but can be separated by up to 4 degrees of driveshaft angle.
see here:
http://www.rustynutscarclub.com/driveline.htm
Jag PL diffs require a diff lube specially blended for limited slip diffs. Back a ways the recommended oils were Castrol Hypoy L.S. , Shell S8096B , BP Limslip 90/1 , and a few others. Mind you, this is from an old workshop manual so many may no longer be available and you'd have to seek equivalents. Although I'd suggest a good Hypoid 90 with additive would be OK.