2005 Chevy Silverado 2500 HD 6.6L Duramax: Fixing P0602, P0700, U0101 and Transmission Reverse Issues

Hello, My uncle owns a 2005 Chevrolet Silverado 25-00 HD with a 6.6L Duramax diesel engine. Two years ago, the truck experienced a transmission failure — it could move forward but not reverse. He replaced the Allison 1000 transmission, but the issue has returned. Recently, a mechanic attempted to resolve the problem by adjusting the 4WD buttons, and for a short time, reverse gear functioned properly. However, this functionality eventually disappeared. At times, the truck also fails to engage forward gears. An OBD-II scan revealed several fault codes: P0602, P0700, U0101, as well as dashboard and transfer case-related codes (specific details are unclear). I’ve researched online and learned that troubleshooting this vehicle requires the truck’s systems wiring diagram. Where can I obtain an official or reliable source for the 2005 Silverado 2500 HD wiring diagram? Additionally, where are the HS CAN+ and HS CAN- connectors located on this model? My uncle informed me that he replaced the Allison ECU-TCM approximately 1.5 years ago. Any technical advice or guidance for diagnosing and repairing these issues is greatly appreciated.