Have '92 sierra 5 speed and when setting idling the speedometer keeps jumping back and forth. I get a feeling it's some kind of sensor somewhere. If it is where is it located? Thank you!

The vehicle speed sensor is located at the transmission tail shaft housing.
Unplug the wiring harness to it, remove the retaining bolt, remove the sensor from the tail shaft housing with a fluid catch pan underneath, swap the "O" ring unless the new sensor comes with a new one, and reverse procedure to install.
Check transmission fluid level and fill if necessary.

Good luck!