Why does my transmission 'howl' or whine a lot when slowing down?

According to Terry Livermore in the July 8, 2000 Digest:

"Winding down or "coast down" whine or howl noises that go away when the accelerator is depressed are often caused by a loose or worn drive pinion bearing in the rear differential. These are usually loudest between 45 to 25 MPH. I once pulled the transfer case and driveshaft and drove a little to make sure before I decided where mine was coming from."