Transmission

I have a 1994 chevrolet silverado 1500 4wd truck with overdrive. It suddenly stopped shifting into second and overdrive. There is no clutch material in the pan, and the truck runs good in the other gears. Someone told me about a shift solenoid on the transmission being bad, as it happened to them. I am thinking this may be the case. Is it located on the side(top) on the passenger side? Think that may be the problem? Where can I get one, or is it a "dealer item"? 5.7 liter (350) 117000 miles. Fluid levels good.