Why does is my transmission not operating above 1 - 1 1/2 RPMs?

Why does is my transmission not operating above 1 - 1 1/2 RPMs? I have a Chevrolet 1996 Silverado 1500, when I put it in drive the transmission pulls but once I get above 1-1 1/2 RPMs the transmission doesn't pull. The RPMs keep going up but my speedometer goes down. Any suggestions as to what to check?