Snoman, Don't quote me on this, but I understand that newer GM Trucks/SUVs are already coming from the factory with full synthetic gear lubes installed. The GM Parts Counter folks could probably confirm this.
One fallicy/belief I had many many years ago, was that you never were supposed to completely change out gear fluids, and was supposed to only just top them off.
I probably got this dumb idea working-servicing Semi-Tractors, where it would be prohibitly expensive for a company to constantly be periodically changing out gear lubes on their large fleets of trucks.
I understand now, how untrue that is, and it actually wouldn't hurt a thing if you hypothetically changed out axle lubes every 3,000 miles, but as we all know, this would be an excessive waste of money, and is unnecessary to do.
Typically, what I've done over the years with many of my vehicles, when I felt it was time to change-refresh Gear Lubes, was to just use an inexpensive Fluid Suction Gun, with a 2' Hose attached to empty Axles, and have a Oil Drain Pan nearby to empty the old Lube into. Takes only about 5 minutes per Axle to do, and another 5, or so, to refill.
While I know I don't get every last drop of Lube out in an axle, which I feel is not that important/critical, It saved me the hassle of ripping off Axle Covers, and worrying about the chance of minor leakage down the road. Mark