I was under a Lincoln Towncar this evening, pulling the 5.0 for another project and was dropping the driveline and looked at the rear end setup.
Guess what its identical to the front of our XJ's.
Some cutting and welding it would be a cheap way to get a Quad link on the rear of the XJ.

Maybe the rearend out of mid 80's Continental for the disc brakes and a Ford 8.8 would be an idea with Ford Control arms to convert an XJ to coils.

What Do you think?