The voltage regulator is controlled by the powertrain control module and it is located on the passenger side firewall on most Dodge 3500 diesel. This is a problem that some people fix by installing an external voltage regulator.
The voltage regulator is located in the Single Board Engine Controller (SBEC or ECM). If you have to replace it, you have to replace the computer. The computer has the voltage regulator in it. The alternator gets feedback from the computer to produce more of less voltage.