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.