Just a quick note regarding my experience replacing the diff in my car ('91 3.2 auto).
Everything went to plan (with help from Don's step-by-step guide - here's one of his pages on JL http://www.jag-lovers.org/snaps/snap_view.php3?id=1361926346)
My diff had only done 63k miles but was suffering from a grumbling pinion bearing. This was a GKN diff and the replacement (a Salisbury unit) was obviously from a later car because it didn't have the hall effect rotor on one of the output shafts for the speedo - I just swapped the rotor, spacer and sensor bracket from the old unit to the replacement diff.
The replacement diff needed the pinion seal replacing and, using Don's guide, a long reach 36 mm socket was needed, however it turned out we needed a 1.25 inch (32 mm) and it needed to be extra long reach because of the pinion spigot (that projects into the prop shaft).
Even with this delay in getting the socket, and swapping of a few studs and restoring some threads, my brother and I managed to get everything sorted in a day - 10am - 8pm. Hard work but a good job done.