Is your truck a SRW? 4.10 gears? Tire size 265/75/16?
I have a 94 K2500, 4.10 gears, with 265/75/16 tires. At 2000 RPM I'm doing 60 MPH. Has the alternator ever been changed? If so, you likely ended up with a gasser pulley on the alt. The diesel pulley should be 2 9/16" diameter across the face.
I'll echo Leroy (Burning Oli), fill out your signature so we can know what vehicle(s) you have, and are talking about.