title says it all.. I can get the speedometer and the live data stream to say the car is moving up to 10mph, while just revving the engine in park. I've seen this on another GM car (1995 astro van). Anyone know what causes this? Speed sensor bad?

 

This is likely caused by electrical interference in the VSS wiring. The wiring is supposed to be isolated with a grounded shield in the harness surrounding the wires. I seem to recall a TSB on the fullsize trucks for something similar to this where the interference was caused by worn distributor bushings, but can't find anything that applies to the s-series vehicles.

It could be influenced by a dirty speed sensor sending a somewhat lower amplitude signal to the PCM (or ECU depending on what year your vehicle is) so I would recommend cleaning the sensor before going any further in other directions. The amplitude does not matter for the controls as it is a square wave signal and the computer will count the pulses, but at lower amplitude, the background electrical noise becomes more of a factor.