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