If I remember correctly from back in my early car alarm installer days, the locks were wierd on the 03+ GM trucks. 02 and older power locks were actually controlled by switches and relays. But in 03 (and 02 for the Trailblazer) they switched EVERYTHING over to the new databus computer controlled system. The computer controlled the power locks, windows, pretty much everything. You can connect a Tech 2 and roll each window up or down, test each speaker, and lock and unlock the doors (plus a shitload of other things).
But this was a nightmare for alarm installers to integrate keyless entry into the power locks. You had to actually run wires into EACH door and tie into each lock actuator with two relays. But I remember reading on one of the installer forums I was on, and someone found a wire under the dash that triggered ONLY the back doors. Each front door has a "door module" (or window switch as most people call it) that communicated with the BCM and that module actually locked and unlocked the door. But the rear doors were connected to something up under the dash. I wish I had more information about this, but not long after that several companies released the bypass modules that actually locked and unlocked the doors by communication with the BCM just like factory. So nobody did the relay method after that, everyone just used a bypass module.
So the point of all my rambling is to basically tell you that the front and rear doors are controlled by separate modules. All I remember is the connection for the rear doors was up under the driver's side dash near the kick panel area. It's in that relay block that sits in that corner. If you have not messed with any of your window switches lately, I'd investigate that area.
Another possibility, if you replaced the power window switches with other ones not programmed to your truck, THAT could be the issue as well.
