my 99 gmc jimmy seems to have lost overdrive. the tach revs but the speedometer doesnt move. it will go into reverse.

You may also have a bad VSS (Vehicle Speed Sensor). If CEL is on, see if it will give you a code. The VSS or Variable Speed Sensor, is an Electrical pulse sensor that sends a signal to the DRAC (Digital Ratio Adapter Control) informing it of the vehicals speed. From there signals are sent to the Power Train Control Module, and to the Cruise Control. If your Speedometer is not working, 99% of the time its caused by a defective VSS. On 2WD models, the VSS is located on the tailshaft of the Transmission, drivers side. On 4WD models, the VSS is located on the tailshaft of the Transfer Case, drivers side. You will need a 1" socket to remove the sensor.