The heater hose has an air lock and needs bled. The same thing happens to my '94 3.9 Dakota. One of the heater hoses loops way up and over the engine and is prone to air locks. I cut the hose at the top of the arc and installed a Prestone flush & fill tee. If an air lock develops, I loosen the cap and let the air out.
This has been in the back of my mind. The truck has exactly what you describe already installed in the upper heater hose. My hands were too dang cold to loosen that cap the other day so I hoped for the best. I'll put more effort into that when I get back to it.
My other thought was maybe the impeller on the pump is rotted away and I'm just not getting enough "flow" for the heater.
