I had researched this a while back. The XJ seat belts do not fit the MJ. For the 4 dr., the retractors sit lower in the body in an XJ. Plus, it is shaped differently. For the 2 dr., since the front seat tilts to allow rear seat entry, those too are positioned differently, also.
I found a company that repaired mine. I will have to look up the name.
(On mine, the belt stopped retracting.)