Rear Noise Mainly On Deceleration

Sounds like someone has installed aftermarket gears and doesn't have them shimmed up correctly.

It could be any of the above. It's hard to diagnose a sound problem that is explained with written text via the interwebs.

From my experience: The bearings will squawk like a little bird when they are going out and usually do it when letting off the throttle until they get so bad, then they will do it non stop (at that point it's almost too late).

If you get what I describe as a howl/growl etc when letting off then it typically (not always) means the gears are not shimmed properly or there is another gear related issue.

Find a good gear shop (4x4 places are usually very knowledgeable) or ask around to some folks to let a mechanic properly diagnose.

Again, my disclaimer is this is the interwebs and I cannot possibly predict an accurate outcome but a wah wah wah screams gear set up to me. It's more of an annoyance and could potentially cause premature wear but likely not detrimental to your car.

Diagnosing Rear Differential Noise
There are various situations which can create rear differential noise. For example, howling of gears is a sure sign of wearing. If you notice that the howling noise only occurs during deceleration, then it is a good indicator that the pinion-bearing preload has loosened.

Howling under acceleration at various speeds, however, indicates that the gears are already worn out or are out of alignment or depth with each other. If rear differential noise occurs while accelerating the car only at a certain speed, it is likely because the gears have become worn due to overloading and lubrication failure. If your gears are newly installed and still create a howling noise, double-check its preload and make sure that the teeth are properly aligned.

Rumbling and whirring noises at speed over 20 mph, moreover, can be the result of worn carrier bearings. For vehicles with C-clip axles, the rear differential noise may change at different turns. Generally, worn out pinion bearings can create whirring noises at various speeds, be it may during deceleration and/or acceleration. If the pinion bearings are the problem, they create more of a whirring noise than a rumble because it turns several times faster than the carrier assembly. Regular clunking every few feet can also be an indicator of a broken pinion gear and/or chipped and damaged ring gear.

Overly worn out bearings tend to make a howling noise when they do not properly support the gears. Rumbling while turning, on the other hand, is a sign of bad wheel bearings. Clunking and banging noises on the corners can be due to lack of sufficient posi-traction lubrication, broken spider gears, or worn posi-traction or limited-slip clutches. Broken spider gears, moreover, can also immobilize the differential and create a loud, crunching sound during final departure. If the rear differential noise is characterized by clunking every two or three feet, then there is a great chance that a broken ring gear is the problem with the section with the broken teeth banging or grinding as it tries to engage the pinion.

Having a chipped or missing high spot on a gear tooth sounds closely similar to a broken gear except that the rear differential noise only happens while accelerating or decelerating. This is because the problem is just present on one side of the offending tooth. This rear differential noise is described as a heavy clicking type of sound which occurs every eight feet or so. If the pinion is the one that has a high spot, the noise occurs every two or three feet and is much more pronounced because of its higher frequency

I would pull off the cover for inspection and fluid change, only a few bolts and maybe see something(fingers crossed ) in there. No seepage from axle shafts? A leaking seal there can be caused by a bad wheel bearing. The driveshaft itself should have no movement, there is play in the rear end that is normal.