I just joined this club to say thanks for all the info you provided.
I own a 2006 Xterra, 6spd.
I had a problem with humming noise from the rear at 70 mph.
The rear differential was replaced yesterday and the noise is gone.
I will post this issue at Powertrain topic.
Thanks a bunch again.
My X has only 4500 miles and I noticed the noise when it had 2K miles on it. The tech told me that it was a rare case and simply called it a internal failure. I guess it was a defective parts. I posted a reply regarding this issue at Engine and Powertrain. You can check it out under revisiting whining noise.