I would think that any electric connection to the tranny is going to be vunerable to moisture and trail obstacles. As simple as it sounds, I'd suggest crawling under there to clean the connection with electric contact cleaner and check for shorts in the insulation. A little dielectric grease, some extra electrical tape for insurance and a double check of the fluid level in the tranny may fix the problem.

83 mitsu flat bed 2.6l weber, 88 auto tranny, 2" body lift, 4.625 LSD, 235/85-16E mudders (33 x 9.50).