You do have auto leveling if the OE rear shocks have never been changed.
To explain a little more, the G65 RPO code designates Nivomat technology shocks on the rear, and those are manual self-leveling. They are beefy shocks that adjust to the load as you drive. They are paired with softer springs, hence the Premium Smooth Ride (PSR) designation. If you replace them with standard shocks you'll need to replace the springs with either Z71 (non PSR) springs or what Bilstein indicates (199021), and those stiffer springs would work with standard shocks. But you lose the self-adjusting feature of the Nivomats. (I'm not sure if there is a third style of spring for this model year.) Not changing the springs when converting to standard shocks will cause dangerous sway under normal driving conditions.
Is there something wrong with your Nivomats? I use my '04 to tow a rental camper every year, at times without a WDH, depending of course on the tongue weight of the trailer. They level up within a couple of blocks of towing. You can test them by measuring a point on the rear bumper with no load, then tossing it 200-400 lbs of stuff in the back and measuring again, then take it for a short drive of a few blocks. Return to the same spot and measure again to see if they've brought up the back end of the truck to a decent level.
Having said that, I have replaced the front shocks with KYB Gas-A-Just KG5040s and am happy with them for retaining the PSR. Stay away from the KYB Excel-G bottom of the line shocks.
Pic of Nivomat shock:
