Well, the one thing that I haven't seen is anything you happened to notice when you GOT UNDER THERE AND LOOKED AT EVERYTHING BEFORE STARTING A THREAd! But seriously, if you have the sound isolated down to the diff area; the problem could very well be obvious... Just sayin'
Check the nuts/bolts holding your axles to the diff... and the DS nuts/bolts... as well as the other mounting points & the bushings. Could just be some slop due to loose/missing hardware... Also verify the axle nuts are securely fastened thru the rear hubs...
Might be a little easier to help if you can tell us for sure what the problem is NOT... Yano?