The XJ is the Jeep Cherokee, not the "Grand" but just regular Cherokee. The MJ (Comanche) is pretty much the same as an Xj from the doors forward. So, yes the doors, fenders, hood, grill, bumper will interchange. Your comanche being an 87 will accept XJ parts up till the 1997 model year. If you wanted the newer look of the 97 and up XJ some modifications are necessary.
If I'm incorrect in any of this someone will be right along to correct me, but I think I'm right for the most part!
Congrats on the MJ BTW! :banana: Got any pics of it?