GMC Troubleshooting Questions? Ask a Mechanic for Answers
Sounds like a loss of line pressure. If no codes are present for the line pressure solenoid it would be a mechanical failure in the pump or torque converter. A line pressure gauge would need to be installed to confirm.
It's pretty much going to have to be disassembled to see where the failure is. I know pulling it's no fun and hate to be the bearer of the bad news.