Just found a custom 4.09 on eBay built up with later type crown/pinion in the early case with lsd
Nice to know there's options
https://www.ebay.co.uk/itm/JAGUAR-XJS-XJ6-4-09-PL-REAR-SALISBURY-DIFFERENTIAL-RACING-CUSTOM-REBUILT-ETYPE/222548198223?hash=item33d0e8034f:g:bcQAAOSwA3dYIxl i