I did something similar with our old '92 Cherokee. You would have to take
out the back seat completely and I put the platform on PVC "stilts". Enough to put boxes underneath for camping, first-aid, tools, gear, etc.
Each end had hinges -- when sleeping you would put the front seats all the way forward and get enough room to sleep. I also made it with hinged rear with liftgate up. My wife made a little tent-like thing that we
would use if it was wet out or needed to change clothes.
It worked out very well. I kept it and now waiting to purchase another XJ.
Looking for a 2000/2001 but getting a little worried about the infamous
head crack issue ........... Randy from Oceanside, CA