Why can't I shift from 2WD to 4WD on my 1999 F-150?
If its a manual transmission be certain that the clutch is completely down to the floor . But, just to complicate things, sometimes making the shift requires that the gears in the transmission be turned ever so slightly and requires that the clutch be released just enough to get that done.
Its not uncommon for the shift to be difficult with the truck stopped.
How about Asperger's?
Sometimes it's not really apparent that someone has it, because sometimes it's not very advanced. Do you have problem reading other's emotions? Or do you find yourself annoying people all the time?
I mean I'm just guessing here, but that is what I suspect it to be...
You also say that you are intelligent (or you think you are anyways) which was one other clue for my guess.