Just drained the diff and inspected the teeth . (Put the rear axle on jacks so the wheels would spin.) Used spray brake cleaner to get the teeth nice and bright. The old oil was black, dirty and piss thin. Cleaned the two mating faces up and tightened the cover down on going to opposite bolts each time. ( On fresh gasket sealant.) Then I refilled the diff with 75/90 using a large surgical syringe. (fFriends in nursing.) Went out and did a few figure 8 turns to get the oil in everywhere. Guess what ? Popping, clunking noise gone. Also had a chat with a jeep rebuild guy near me and he reckons there is always a little play in the axle shafts. Moral of the story: Do the maintenance before you start buying new diff parts. Thanks for all the guidance on the forum.