I have the same problem with my '03 T&C. Also the rear heater coil gets really hot while the A/C is trying to keep the cabin cool. Engine temperature gauge looks OK.

 

I don't believe there is a shut-off for the heater coil when the A/C is on, The idea is that air should not be flowing over the heater coil when the controls call for A/C. Coolant from the engine, however, is still flowing through the coil.

 

I installed a new thermostat and everything is back to normal. Not sure why the engine didn't over-heat...