Transmission problem? throttle position sensor? tranny computer?

Having a problem with 1985 toyota pickup 4x4. . the tranny info is secondhand from a mechanic and i do not know auto transmissions. all i know is that the tranny seems to run and shift normally till 40 mph or so and then it seems to upshift to the highest gear prematurely and lugs there. then it won't downshift when the vehicle comes under load. ( this is when the tranny guy said it was locking out and not downshifting when it was being told to by the throttle position sensor. )you have to floor it in order for the tranny to downshift. it sounds like it is shifting back and forth from 2nd to overdrive without using or staying in the gear or gears inbetween. this is most apparent while you are driving down the freeway at 45-65 mph and you are confronted with a steadily climbing hill. the truck lugs and won't downshift unless you floor it. so audibly the truck is switching back and forth from a very high rpm and then it lugs. so you can scream up the hill or lose speed while lugging. those are your choices until the hill ends.
during town driving it drives very well. upshifting and downshifting without any issue, until you get over 40 mph. does this sound like the tps? or the transmission control module? got any ideas?