After looking at the pictures on your site, I would be worried about even TRYING to repair an XJ with the "frame" having as much rust as yours seems to show.
Vehicular value issues aside, getting the "frame" straightened with that much rust will be problematic for any competent body shop. "Fixing" the repair WITHOUT straightening the frame properly (the hillbilly way) would result in NO safe towing ability, not to mention potential axle alignment issues where the rear spring shackles attach.
I hate to say this, but your XJ appears to have died a quiet death here. The thing is beyond redemption for repair, and must now go to the great off-road trail in the sky.
The same thing happened to my 1992. A Dodge Stratus hit my trailer hitch at about 40mph. Externally, the XJ looked fine, with no damage to any body part except the bumper. Underneath, the "frame" was kinked up about 2 inches, right where the front of the hitch met the frame. The body shop estimated the VISIBLE damage at $10K, and felt sure that once it was dug into, it would go up. Needless to say, the insurance company totalled it. :bawl: