a roaring noise that changes pitch between acceleration and deceleration is likely the pinion bearing. grab the yoke at the front of the diff (rear end of drive shaft) and rotate it, and wiggle it sideways. you might be able to fee a little bit of looseness, but with all the other stuff attached (axles, drive shaft), you may not be able to feel it.
your 'wheels up in the air' test is valid and the only choices are the center diff (ring, pinion, pinion bearing, carrier bearing) or the outside (rear wheel bearings).
its most likely the pinion bearing with the varying pitch.