FYI - My 06 2WD X is going to have the entire diff. replaced as soon as they get one in. As is usually the case, it's on national backorder.
This comes from a harmonic whine I have been hearing since just about a month or two after I got it, but it may have been there from day one.
It's very hard to duplicate, but it's there in mine.
Basically, if you're running a steady 75 or 80, where I usually run on the freeway, and you are feathering the throttle (not on it, but not all the way off of it), you can hear this slight wine, which can become louder at different speeds. You can also hear it if you have the cruise set at about the same speed, and then hit the 'Cancel' button to turn it off. As the truck decellerates, you can hear this whine momentarily.To give you an idea of what it sounds like, it's like when you're on a school bus, sitting in the back, and the bus gets to going really fast and you just hear this whining groan from the rear.
Someone else suggested that the backlash should be adjusted in the diff., but the Service Manager at the dealer said they aren't adjustable any more.
They're ordering a whole new rearend assembly.

The truck is 5 months and 19 days old today, and has 19,750 miles on it so far.

Boy, you do a lot of driving, that about 120 miles a day on average. I wouldnt want your gasoline bill, thats for sure

Geez that is a lot of miles. My truck is a year old and has 16K. Hope it is not in Georgia Traffic.

Hmmm....I've posted in the past about a similar 'whine' that I hear on my vehicle. It occurs between 64-70mph and only when I have my foot on the gas, or when fuel is being supplied to the engine at that rate of speed/rpms. For example, if I am sitting there on cruise control I hear the whine, but if I start to coast down a hill or something (while on cruise) the noise goes away because the accelerator cuts out. I don't hear it below 60mph or above 75mph or so (although I rarely travel above 70 on the highway) My car has 20,600 miles on it. Could it be the same thing?

Search this forum for "diff whine" and you'll pull up some older threads that discussed the exact same issue on 1st-gen Xterras, too. It appears to be a not-too-uncommon issue.

FWIW, the rear diff on my truck has been whining at certain speeds for about 20,000 miles, but since I'm now about 80,000 miles out of warranty, I'm not going to worry about it.

Unless it bothers the heck out of you or is really loud I wouldn't bother with it. My F-150 has had a harmonic that sets in about 41.5mph and 83mph since day one. 113K and it's still fine.

Your service manager is an idiot, BTW, all the gear sets are adjustable, that's how they are set up to begin with. The problem is that almost none of the service departments have techs that mess with differentials at all. Most of them have never worked on one other than to change out the entire thing (big whoop).