Jeff, my fan switch crapped a few years ago. While it quit working on the heat side it still worked on the AC side. I found out that I could still get cab heat (required in our MN winters) by switching the fan to the AC side and turning the temp knob to the hot side.
If your fan works on the AC settings it is probably the switch. If you order the switch you need to get the one specific to the diesel. Which i think was cheaper than the gas.