Here's the thing that speaks against a blend door with my situation--I replaced my blend door with the heater core 14 months ago, and it had the "improved" metal band around the hinge point. I can turn the temperature control **** in my truck to all points around the dial with corresponding changes in temperature. I can also place mine on Max AC setting, and the recirculating door above the passenger floorboard is working as it should. Bottom line--something happened abruptly to my heat right in the middle of a 3-hour drive. My new thermostat did indeed seemingly help a little, but I'm still driving around with lukewarm air just like you.
I changed the heater core, blend door, and coolant 14 months ago, and drove it all last winter with great heat. And I had great heat right up until it stopped the other night. I am at a loss for what might be wrong. I also started the truck last night, got it warmed up, then shut off the engine but left the heater running with the key switch. As expected, the hot air ran out in about 5 seconds or so, but started back when I restarted the engine (and the water pump). So I don't think I have a clogged core either.
I am as perplexed as you on my truck. This fall, I flushed the cooling system and back flushed heater core. Then I replaced the thermostat. Had a leak I couldn't find, thought it was thermostat housing, took it to a garage and it was a hose clamp. But while there, they also flushed the heater core. I've ran it with the coolant cap off on an incline. And still just luke warm air. It didn't have warm air when we bought it. When I replaced the thermostat I noticed previous owner had replaced it, but he put it in backwards. Thought when I put in a new one correctly, that was the fix. Unfortunately, it had no effect.
They don't have more than 1 thermostat do they?
I'd think if the heater core was plugged you'd get no warm air at all.
After I double-check the coolant or overfill it, if that doesn't work, I may have to resort back to putting cardboard in front of the radiator to see if that helps. Weather is in the teens here and wife is getting cold - it's her truck. We have another 02 with the V8 5.4 liter and it heats up fine - 150K+ miles on it. I've only changed coolant on it.