Mine doesn't go that low either... I wish it did though, well maybe a bit higher.

So FYI- At around 65k miles I was experiencing a very faint diff whine at exactly 60mph. The last visit to the dealer for free CPO maintenance, I had them change the oil and front brakes and I also had them change all the fluids in the F&R differentials, transfer case and the transmission. My slight Diff whine is 100% gone! I figured I would post this in case some of you are experiencing the diff whine at or around 60mph.