As my own follow-up to this issue, I was at the junk yard looking for parts for a different project and I found 2 vehicles with which I could experiment - a '03 Envoy XL and a '05 Envoy XL. I removed a rear door from each vehicle and installed the '05 door on the '03. It was a perfect fit except for the mechanism that holds the door open (2 bolts on the '02-'04 and 1 bolt on the '05-'09). However, those are removable and I confirmed that they swap onto the other door just fine. I also confirmed that the wiring harnesses are identical.
So, I have no reason to believe that the rear doors aren't fully interchangeable for all years '02-'09. If anyone has something that I perhaps overlooked, please let me know and I'll investigate.
I tried an earlier post that included pictures, but perhaps my membership level doesn't allow that, as my post has still not shown up on the forum.