if it only happens on deceleration most likely its the coast side of you ring gear on your carrier or the coast side of the pinion. or a combination of both. this could just be from old age. i replaced a ring and pinion in a car once that looked fine but made a nice whine on any kind of decel.
it could also be the mesh between the gears. a quick way to check it is to put some yellow gear mess paste on the coast side of the ring gear and check the contact patch.
this is what it should look like on both sides. or close.
^this is the drive side FYI
it could even be the backlash (most likely not, you'd get more of a clunking sound from that)
if you take the cover off and move the ring gear back and forth ever so slightly to feel the backlash in the gears. A good rule of thumb is if you can feel it move and hear it move but not see it its pretty close. now this is NOT the correct way to adjust backlash. you could buy a feeler gauge and check it as well.
personally it sounds like a ring and pinion to me.
let us know what you find it to be. good luck
this also sounds like a u joint problem at those speeds but you wouldnt get a whine from it. it would be more of a vibration. an easy way to check for u joints its to drive it with out the front drive shaft.
Race Car - 1998 BMW 328is, GC Race Coilover, Nitto NT01's, Roll Cage, Gutted, intake manifold, under construction
Daily - 1998 TJ L6 auto, 4" RC Lift, Custom Front Bumper, Badland Winch, 32" tires, Congo Rack, other odds and ends