actually, the towing capacity of an xj is only 5000lbs.
i know 6000 doesn't seem like much more, but it can push the rear end of the jeep around when cornering.
also, if DMV happens to pull you in the scales, you'd be overweight and can be fined.
just remember, there is a 500lb max tongue law.
if your xj is equipped with a factory installed hitch, it should have a mount in the middle to the body as well as bolted to the frame provided nut strips.
i know this link is for a 2000 model xj, but the weight is still the same.