95 Rodeo tranny, no 4th/ just overreevs?

4 speed Automatic

Problem: No 4th gear (overdrive)

seems like an electrical problem to me, but i'm afraid it could be the transmission

Paid $800 for the truck, getting another trans would be like $300
and i can't use my truck for long distance driving, being limited to around 45 mph, w/out high-revs that drain my gas tank

anyone have access to tech information that i can use?
or does anyone know where the sensor is or how i can find it?
has anyone had to deal with this problem?

I've seached the forums, but it seems the question was asked but never answered. Thanks for any help given.

~UD