In answer to your question about gear oil....just regular 80w-90 will work.
Thanks for the advice!Looks like a 7 1/4, you have plenty of options to swap, ford 9'', ford 8.8 the 8.8's seemed to get a little expensive thanks to the jeep guys. a quick find would be Chrysler 8 1/4 found in many of the nearly 3 million xj's made. You may come across a 44 in an xj but those are hard to come buy because of guys like me.