Should be fixed for good now.
Removed the airbox, coolant/washer tanks, and cruise control. Removed the four screws holding the blower motor in, removed and unplugged it. Removed the blower motor resistor (that I just replaced two weeks ago...ugh).
The bearings/bushings were shot. It had noticeable drag/resistance when you tried to turn it. I think this resistance was causing the fan to pull a lot of current, which overheated the thermal fuse in the resistor assembly, causing me to lose all speeds but high. Eventually, I think the bearings got so bad that the blower motor wouldn't turn at all.
I took a peek at what I could see of the AC evaporator coil. DIRTY. Never been cleaned in 324,000mi. I got a hose in there and flushed it out the best I could, and the box drained nicely.
I installed a new Motorcraft blower fan and a new fan blade from Continental. This was the easy part.
I didn't want to buy another blower fan resistor because the one in there was brand new, and only had a blown thermal fuse. I ordered a Radio Shack #2701320 139*C thermal protector fuse, which is only a few degrees lower than the OEM fuse. Close enough, I figure.
Checked, and sure enough, the thermal fuse was blown. No continuity, while the new one had continuity.
Changing this...sucks. I initially planned to solder it, but I was concerned the heat from the iron would blow it out. I managed to use a pick and small screwdrivers and such to remove the old one, enlarge the crimp holes, and insert the new one, and re-crimp it with pliers. I was only able to engage one hole on each side, but it felt pretty solid. We'll see how long it lasts.
Works like a champ now!