UB - I've been thinking about that for a little while now. Doesn't the air just get passed over a coil to heat it up for the defroster? Heating air ALWAYS dehumidifies it, and cooling air ALWAYS increases the density which increases the humidity - thus you see water dripping from air conditioners because they are condensing and removing that extra humidity. I need to look at why the refrigerant operating (A/C on)(normally used to cool the air) would help dehumidify when the heat is cranked up when it normally increases the humidity when it's being used to cool the air...
You almost answered your own question. As the heated air passes through the AC evaporater the moisture gets stripped off by the cold fins. (much the same reason your window fogs!) It drips out the bottom of the truck instead of blowing back on the windshield. If you have access to an ACed vehicle that you can disengage the ac when on defrost then experiment. Crank on the defrost without the AC on. As soon as the window starts thawing the rest of the windows condense because of the cold window and warm moist air....now hit the ac.....fog will disappear in seconds!