The Nissan Path

I have a 1997 Pathfinder SE that has a whining noise coming from the back half of the vehicle. I here it while driving. If you take your foot off the accelerator it stops but comes right back as soon as you press the pedal again. Any help with this issue would be greatly appreciated.

If it does it when in park or neutral I would guess the fuel pump.
If it's doing it in drive under a load (accelerating) I would guess something in the driveline or a bearing somewhere.

It's probably the rear diff; I replaced a few of them when I worked for Nissan. They'll make a high-pitched whine with a load on them, but when you release the gas pedal and take the load off of the diff, the noise goes away.

Any idea how much it would cost to fix the differential or what part is making the noise ( ex. bearing, gears, etc.) Thanks.