transmision will shift fine except into drive stays in secound I believe that it is the speed sensor
how to repalce a speed sensor on 1993 GMC Vandura 3500
The vehicle speed sensor (vss)is located on the side of the transmission on the driver's side. It is held in by a metal bracket that is part of the vss assembly. Unplug the electrical connecter, remove the one bolt on the bracket that holds it in, pull it out noting that there is an "o" ring seal to prevent fluid leakage. Reverse to install the new one. Note: The vss sends data to the ecm which in turn sends data to the speed gage on the dash, and information to electronically shift your transmission. If the vss is defective your speedometer should stop working and your tranny will not shift out of second. The service engine soon light on the dash should light. A code reader will reveal code 24, vss open or shorted. Trannies that are electronic shift on the Vandura are the 4L60E and the 4L80E. If you go to the part store to purchase one you will need to know if the pan on the tranny has 16 or 17 bolts. Hope this helps you.