Took my '06 in at about 4K with a whine that drove me nuts at about 35 and again at 55-65 mph. Driving in town 90% of the time the whine was a constant. I had the service manager drive it and got the the standard answer... sitting closer to the rearend, you would hear more noise. He did agreed that the whine was there but referenced the GM memo that advised us that it was "normal". At 8K I took it back in. Played nice and took another drive and the service manager. He agreed that it was more pronounced than before and annoying as hell in freeway traffic . They installed a new L/S diff. the next day and the service ticket noted "ring & pinion gears failed causing whine". Sadly I have much the same whine as before but it's at a much lower decibel level... for now. I don't know if this helps you much... but you have lots of company.
There are three distinct problems I see here that fall into two categories. The first category is gear whine, I've experienced this on other cars. If you're into mods, then you've probably had differential gears changed at one point or another...I've had it done in my Mustang. Anyway, improperly set up gears will typically whine, although, some brands will whine even if set up properly. These differentials seem to be particularly loud.
Your differential didn't fail, it just had a noise. If you had experienced bad whine throughout the RPM range or some other symptoms, then that would definitely had been a sign of trouble. As it is, I have gear whine from about 45 mph to 65 mph. I guess it was annoying at first, but I've gotten used to it.
The differential is not failing, so I'm really not inclined to do anything about it. Then again, NVH problems don't really bother me, nor am I sensitive about squeaks, rattles, clunks and the like. I love to listen to music when I drive, generally, I find that Dave Grohl is much louder than my differential!
The second category is out and out failure of parts. That usually takes two forms. The first is a leaky pinion seal. That's easy enough to detect. You'll notice a puddle on the garage floor if you have it.
Another common failure, which I experienced personally, was a failure of the clutch packs. You'll notice it when you make sharp turns, it sounds a bit like rubbing or binding and you might be tricked into thinking it's coming from the front of the car. If you have it, then you will know it, trust me. The sound is very noticeable and will be pronounced during sharp, low speed turns, as when turning into a parking spot...or doing donuts in the parking lot.