the reason for the low tow rating is not the power or brakes etc of the TJ it can pull a hell of a lot more than the 900kg the problem is the wheel base is too short to tow much
A heavy weight behind a short wheel base means it will be pushed through corners or if the trailer gets a sway up it will pull thr short wheel base around with it. Just think of a short and long leaver and you will see why
It is the manufacture that decides on the limit and I think Jeep have been a little too conservitave but I guess in a sue happy society it pays to be conservative in things like this