[Maxo]

My girlfriend's '95 had the same whine so she had a shop take a look at it. They indicated that the noise was coming from her "transfer case". Knowing that they probably meant the front differential she drove it home and we checked the front diff... completely dry.
She posted on here wondering what to expect now and was told that she has a bad bearing now and that her front diff probably won't last very much longer.
We put in mobile1 75w90 full synthetic and the the whine has quieted to the point where it's barely audible.

Her diff whine only occurred when she was off the throttle. The fact your diff whine occurs under load makes little difference. Metal shavings in your fluid may be a good indication that something isn't right.