View attachment 86772
This is how mine was attached when I used to carry it. I built a square tube frame for the cargo area so I could mount stuff in there.
What size tire is that? I had my 37 in the back for a bit and it stood up perfectly between the floor and ceiling, actually have a tire tread mark on the headliner to this day. (yeah I know I still have carpet, a/c and a headliner...)
(This is where I would rave about RidgidCo's bumper but the OP said he has a bumper already.)
You can buy beefy spindle kits for not too much, the DaStaCo latch seems to be the best one.
With an xj you gotta spread the load, it may be time to look at adding some stringers, etc to your design.
I can't find a link now but there is a guy who has a nice xj in Reno (silver I think, I think he put a v8 in it?) but he had a nice rear bumper and it started to tear the "frame" He thought it was from the weight of the tire.
Nathan
99 xj with pretty much everything
http://www.rme4x4.com/showthread.php?83436-Nathan-and-Tanja-s-99-XJ-Build-up