What could a high pitched whining noise from rear end of my truck be?

[Adam E]

I have a 1999 Jeep Grand Cherokee Laredo 4.7L V8 everything stock. I have a high pitched whining noise and it sounds like it's coming from the rear end. It comes and goes, it seems like after it sits for a while that the sound kicks in. It usually kicks in around 20 mph or so and gets louder as you go faster, then as I decelerate the noise slows and gets softer until I'm at a complete stop. I haven't done anything to the rear end, although I did have the transmission fluid changed today. Please help me!!! thanks so much