Which Vehicle Speed Sensor?

I just got a 1990 camaro RS with an automatic tranny. The speedo has been pretty jumpy since I got it. I am pretty sure that the sensor is on its way out, but which one do I need? I see two listed at autozone and napa.

Look at your RPO codes in the rear locking storage compartment. That should tell you which one you need. D81, D8N, D8G and/or D8M are the RPOs that deal with the VSS.