Although I can't prove it beyond a reasonable doubt there is some thruth to the electrical gremlin theory.

I had the same issue with my 95 Passport. The light would come on everytime I would pull a big hill. Clear the code and the light would go out only to come back on later.

I replaced the tranny at a total cost of roughly $2,500.00 used through Phoenix hardparts (don't go through them, long story but absolute nightmare). It took two days replacing the tranny. I also installed a hayden secondary tranny cooler, a big one, I don't remember which one but I definitely overkilled it. The first time I went offroad and placed the T-case in the 4x4 the tranny light came on. I didn't drive the vehicle much so I just blew it off, but on the occassion that I took it out it happended over and over. Finally the alternator punked out on me. I replaced it and wouldn't you know I can't get that tranny light to come back on anymore.

You can always buy an alternator, install it, see if that fixes the problem, if not take it back, or keep it as a trailspare.