Realistically, it totally depends on what kind of use your XJ will see.
A mall crawler rig with the back portion cut away would likely be just fine. Someone who wheels easy to moderate trails and carries alot of gear, spare tire, and stuff would probably see some flexing of the unibody - and eventually cracks and tears to the flexing areas. Hardcore rigs are gonna have problems right from the moment they first get the rig flexed up.
Many moderate to hardcore XJ wheelers will be able to show you cracks that have developed in various locations on their un-chopped rigs. Typically to the upper inside corners near the weather stripping of the "C" pillars, the upper inside corner of the "A" pillar where the roof meets the pillar, and inside the door jambs where the black plastic vent is. The older the XJ, the worse it is. There were some unibody improvements done to '93 and newer XJ's and then again in '97 (If I'm recalling correctly....)
So, basically, the majority of people with experience are gonna tell you that it's definitely not a good idea to cut and remove any structural part of the unibody without reinforcing the area. A cage isn't the only way to reinforce either. A person could still cut the back SUT style and add some structural bracing that would add some strength. But ideally, you want to be able to keep the suspension forces at the rear spring mounts from pushing, pulling and twisting that rear part of the unibody to the point of structural failure.
Just think of the back half of a chopped XJ as a typical pick-up bed. We've all seen what those beds will do when the frame on the truck is flexed - the part of the roof that you want to remove is what's keeping the rear from twisting and flexing.
Ya dig?
