The xj6 and xjs diffs are basically the same, at least internally. They are dana 44's using same gear sets as other d44 applications. They are very strong units and are clean looking with disc brakes mounted inboard near the center section. V12 cars had posi units and others had open diffs. From around 82-87 they used 2.88 gears and earlier years had 3.0somethings, 3.54 and some I believe had like 4.11. Mounting info I don't have but hope this helps some.
Bob