Why is my heater not working?

Did you try flushing the heater core out yet? Could be partly plugged. It can plug and your temp gauge won't show any difference.

I had to replace the heater core on my 98 5.9 limited a few years back and no issues since. We get some decent cold spells in Colorado.

I am trying to compare my 98 with the inline 6cyl 4.0L to yours. So I don't know if the control valve is inside under the dash like mine or not. The reason I suggested about flushing is it not only would check the heater core but also the valve and the hoses. Which I have had one where the inside wall separated and collapse shutting down the flow. Just a thought.