I have spent the last few hours reading up on this and can't find a sure thing answer on it so here is what I have so far:
I will be swapping a XJ HO full dash and harness into my renix 2.5 MJ. I want my full gauge to read right. SO, the part numbers for both the 2.5 and 4.0 renix fuel tanks are the same meaning the fuel pumps are interchangeable. HO 4.0 and renix 4.0 mj tanks part numbers match meaning I can put a HO pump and sender in my renix tank. Great. I have the dummy light cluster in my MJ so the XJ cluster will be coming over too which means I will have the correct gauge to read the sender theoretically. I will be modifying the MJ rear harness to correctly mate up with the rest of the XJ harness.
So the question of the hour is: Are there any big wiring differences that won't allow this to work? I couldn't find a total confirmation on this but someone has to have done it.
This is the MJ to XJ harness splice I plan to do:
http://comancheclub.com/topic/18588-xj-dash-harness-to-rear-mj-harness-wiring-guides/
Someone please confirm this for me!