You can use LSD's from Mk2 through to XJ6 series 1,2 & 3 and all XJS and E type 420G, just about anything from 60's to late 80's.  all V12 XJS models had LSD's.  I got mine from Black Country Jag Dismantlers in Stourbridge, they have hundreds of axles all marked up so you can choose the ratio you want. They were advertising some new old stock powerloks recently from XJS's at £250.

If you get one from a solid axle it is a straight swap, but if you get one from an Independant Rear Suspension model, there's a small modification required to it to fit a mushroom shaped stop in the sidegears to stop the end of the halfshafts bottoming in the side gear.  Ian Jemison from York fitted mine and set it all up for me.

David