Start calling Chrysler, I seem to remember there was a bad batch of heater cores. I can't be more specific, it's just one of those things I remember hearing about.. run a dejanews search because I seem to remember hearing about it on the rec.jeep+willys news group.
Have you tryed pressure checking the cooling system, sometimes if it's a very small leak the coolant comes out as steam when the vehicle is at operating temp. If you check the system cold you may be able to find out where it is leaking, if it is inside or out.