You will need to swap over the engine bay and dash harness, you will have to use the XJs gauge cluster. For the most part, it will be like pulling it out of one XJ and putting it into another. Engine bay/front suspension/dash between XJ and MJ is the same, unless the MJ was an automatic and has a column shifter.
ALL XJ/MJ crossmembers are the same.
The XJ tank will work if you put it in the bed. I have done that exact mod and works fine. However, the MJ sender will not work properly. It will send fuel and the motor should run, but the Renix and Chrystler level senders are different, and using a Renix sender on a Chrystler system will make your fuel gauge read incorrectly. If you plan on keeping the MJ tank, you'll need to get ahold of a very rare 91 or 92 MJ sender.
Does the MJ have driveshafts? If it was originally a 4.0 auto 4x4 then the rear shaft will work. Otherwise you'll need to have it lengthened/shortened.