1991 GMC C3500

Most likely it is the speed sensor, here are the steps how to diagnose it. VEHICLE SPEED SENSOR TESTING & DIAGNOSIS Sensor Resistance Test Disconnect vehicle speed sensor harness connector at transmission, and test resistance of sensor. If sensor resistance is 900-2000 ohms, reconnect harness, and go to Functional Test. If sensor resistance is not 900-2000 ohms, replace vehicle speed sensor, and recheck system. Functional Test Lift and support rear of vehicle. Disconnect instrument cluster connector. Start engine, and place transmission in Drive. Using a voltmeter, measure voltage between instrument cluster connector terminals B13 (Purple/White wire) and B16 (Light Green/Black wire) while varying engine speed. If voltage varies with engine speed, vehicle speed sensor is okay. If no voltage is present, check wires for opens and connectors for tightness. If still no voltage is present, replace vehicle speed sensor.