I have a 1981 2WD GMC Sierra Classic automatic and I just got the engine replaced yesterday with a 350. When I drive the vehicle, it seems like its not shifting into second gear. The engine revs really high and just stays in 1st gear. Eventually after awhile the truck makes a clunking noise and lunges forward and finally switching into second, but stays in second. Before this it just reved really high and changed, but after I filled up the tranny fluid after it was very low, this problem was produced.
I know for a fact that there is no leaking either. Where could the fluid gone : P
Sounds to me like your kick down / TV cable is out of adjustment. If you have a Haynes manual, there are simple instructions there that could help you adjust it. Otherwise, stop by the dealer or a good tranny shop and they should check the adjustment for you.