You've heard it before because it's true.
My guess is an importer or upfitter swapped the axles prior to import. I'm not aware of GM making any 8 lug half ton GMT400's and I've been around them since they were first introduced.
Do you have the option codes available? Should be in the glovebox...
Yeah, sure do. More importantly, I literally could not care less about what other people think of my choices. I have never, and will never build a truck to get anyone else's approval.
Pro tip: hook a battery charger to the truck.
YOU MUST WAIT 10 MINUTES for the light to stay out or the process will not work. Not 9 minutes and 30 seconds. This is where most people screw this up. Also the 10 seconds between cycles is important as well.
Security relearn will take about 30 minutes, that's your short term fix. It WILL happen again.
The only permanent fix is to get the VATS tuned out of the ECM.
The second row seats are different between Suburbans and the Tahoe/Yukon twins. The fronts are the same and will bolt right up, but the second row will need to be frame swapped.