1994 chevy silverado

hi, i have a 1994 chevy silverdao 4x4 5.0 and the transmission is not shifting into overdrive. i was driving about 60mph and all of a sudden it kicked down a gear and the red brake light came on also. about 10 miles later the light went out and it shifted into overdrive. it was fine for about a week and now the brake light is on constantly and has no overdrive. i have talked to a few people and no one has heard of this. ohh the truck has 128k if that helps. any help would be great and i would be very thankfull