Problem solved. I went out under the hood and tapped the green/white wire in the Cruise Control harness. The AVIC-N2 recognized the VSS instantly.
This leads me to suspect that (1) there is a connection problem with the green/white wire in the Radio Harness, or (2) General Motors has run that segment through some filter for exclusive use with their factory radio.
So, bottom line for 2001 Suburban is go to the Cruise Control harness and it's an easy splice.
Best, Dan