UPDATE:
Well for the good or bad Uhaul did end up putting on a class III hitch, I installed a large-ish (18"x9") tranny cooler and changed the fluid, replaced my tired tranny mount, and checked out all the fluids and brakes. A 1999 XJ (4.0L AW4) towing Uhaul tow dolly made it ~2,700 miles without incident. I wouldn't highly recommend it though. Maybe it is just me but she feels tired. My rear springs have sagged a bit and overall it is just a long trip with an XJ. But it did it, brakes were OK (I'm very glad I just replaced everything in the front end), power was OK (chugged a bit uphills), and the cooling system was just fine. The trailer combo didn't push me around too much although wind and trucks can make it a bit floaty.
Again, it is highly possible in a pinch, but if you love your XJ and you have something bigger to tow do it.