The front suspension parts will swap in with no problem.
As noted, the MJ is spring under, the XJ is spring over. Plus, the MJ rear springs are 5" longer than XJ springs. If his lift is an AAL, you can probably use it in the XJ and it'll provide more lift than it does in an MJ. If it's complete spring packs -- they won't fit the XJ. The comment about the rear shocks is also correct -- they won't work. Upper mount on the XJ is bar pins, on the MJ it's an eye and bushing, the same as the bottom end. I took a set of stock XJ shocks and removed the bar pins, thinking I could use them in an MJ. They were an inch too short to even bolt up. (Both vehicles were stock.)