I'm just a bit concerned that it may not actually be overcharging. Have you actually had any electrical problems?
The sytem can run at around 14 volts on the gauge if your battery is a bit low on charge, if you have no real electrical symptoms, you really need to get the engine controllers (it has 2, the Jeep one which controls the alternator function is under the hood, then there is a Bosch diesel controller inside the car) scanned for fault codes. The car is OBDII compliant, so any OBDII scanner should be capable of picking up the codes. You can even get them in Halfords now.