Somis,

Here is a link for OBDII codes:

www.obdii.com

General Motors U2106 Lost Communications with Transmission Control System

I would check the connectors to the NSBU (neutral start backup) switch to make sure they are secure. The NSBU switch is mounted on the driver's side of the Allison. If you click one of the links in my sig, there is an album with pics of the NSBU switch.

If the connectors are secure, you MAY have a bad NSBU. I replaced mine as a preventative measure since the BLACK old style NSBU switch has known issues, and the new TAN switch is an improved version. The part# for the new switch is clearly displayed in the pictures and was about $54 at my local Allison dealer.

I'm going to move this topic to the 2500HD/3500 HD Trucks & Drivetrain forum, since your problem MAY be more appropriate for that forum (drivetrain related). Good luck.