While I know the dealer did something, I cannot yet prove it. I opened a case with GMC, and they dug in and spoke with the dealer. GMC tells me that "dealer said that they did not do anything to the door locks or anything that could effect the door locks so there is nothing we can do." So it looks like I am on my own.
I pulled the tailgate cover off and test the power going to the locking tailgate actuator, and there is none- so I have confirmed its electrical related, and not just 4 bad actuators. I have also discovered that the passenger door lock/unlock control works the driver door, but still not passenger door or other rear doors.
I wish I knew more about these wiring of these trucks so that I could dig deeper, but I am pretty much at my intellectual limit. Ive check all exposed wiring harnesses to ensure they are firmly plugged in. I am not sure what else to do besides taking it to dealer again.