What year is your XJ? I've got a '95 and it as some plugs in the floor underneath the carpet. Do you have access to a welder? Drop the crossmember (support the tranny) to get room, slowly weld on a the end of the bolt. be very careful not to weld the body. You can try using vise grips to twist it the bolt out or after the stud is there, plug weld a nut on. The heat developed from welding with help with the removal. Also it might take a couple times to get it out. Also soaking it with PB blaster/keroal a couple time before won't hurt.