Maybe the hing where the door attached to the heater unit needs a little cleaning and some fresh white lithium grease?
Hmmm... You see the two white plastic bits in Baileysims's photo above...? The piece with the gear teeth meshes with a small gear mounted on the end of the blend door shaft. The best I could manage was to try to squirt some WD-40 behind the gear and onto the shaft - in hopes that perhaps the shaft was seizing up and preventing the blend door from moving easily - but unfortunately it did nothing to free it up. It's all I can do to twist the gear with my thumb and forefinger. No idea why it suddenly became reluctant to turn last year, it's always worked fine since new.
I did first remove the two pieces Baileysims removed, and likewise carefully cleaned, greased then reassembled (along with the WD-40 shot in the dark) but nope... When I turn the temp control knob to either full heating or cooling, the drive motor will move the assembly only a few degrees before the meshed gears begin slipping with an audible "pop, pop, popping" noise.
Only way I can get either full heating or cooling is by "helping it along" by hand, which means lying on my back with legs out the driver's door, and with key on, reaching up and twisting the temp control knob this way or that with one hand, while likewise twisting the blend door shaft gear with my other.
Since it's usually either really hot or really cold where I live, I do the manual "switch" beginning of the season and just live with it. Honestly only takes about 45 seconds, but still a PIA, lol. One day I'll sort it out... Thanks for the suggestion, btw, but this assembly is located immediately under the center dash and I see no way of opening up the blend box shy of removing the entire dashboard first in order to even get to it.