I would think it is speedometer cluster, unless you have had some recent repair to the area where the VSS is (exhaust etc.)
There have been an abnormal amount of problems reported with the trailblazer/envoy instrument cluster.
Have the car scanned with a portable scanner in live data mode, look at speed (MPH) data coming into PCM, If normal then the IP, if abnormal, then I would replace the VSS.
Usually when the VSS fails, the speedometer either reads 0 or the display is eratic.
Disconnecting the negative battery will probably not help, but can't hurt either. Check the VSS for any damage, it may have been hit by a stone or some other debris.
Monday, January 19th, 2009 AT 9:04 AM