Do you have anything in the speedo/odo LCD? If yes do your CCM checks from the DIC and see if it confirms them. I'd think yes. Everything pretty much addresses shorts to ground or an open circuit in the CCM.
The DIC diagnostics routine is here:
http://tech.corvettecentral.com/2011...trouble-codes/
Connector issue? Has the cluster been removed recently?
I'm guessing since the 4L80E has been there for a while that they did function at one time or another. Yes? To get the speedo/odo display operating originally I'd guess they needed to use a DRA to establish the signal OR there are tail-shaft housings that have been modified to use the mechanical gear drive/driven VSS. Which is used in your car? The 4L80E never had a mechanical drive/driven arrangement.
That's where I'd start - whatever was done to get the signal from the 4L80E to the CCM.