The project xj I recently purchased is currently only rear wheel drive due to previous cobbling. It has a 3.05 in the rear d44 and a 3.55 in the front d30. I would like to switch them both to 3.73's or 3.90's.....Question is, what's involved? Swap the whole diff? Sorry if this is a stupid question, just want to be sure before I start buying parts. I searched and didn't find a definitive answer. Thanks for any help.

you're better off to get that d44 regeared to match your front than to get a d30 with 3.07s to match the rear.

or just regear both axles to 4.56 and be done with it. however, that is quite expensive.