You're seeing the aftermath of a pinion bearing failure, or at least the pinion nut coming loose and falling off. The first time the driver backs off the throttle after losing the pinion nut, the pinion gear gets sucked back into the differential carrier. The noise it makes when that happens is horrendous! It looks like somebody probably cleaned out the shavings, put a new pinion nut on, and drove on. The fact that the pinion nut is loose again probably means you've got one of the pinion bearings failing slowly and letting the nut get loose. I'd recommend a complete teardown of the rear end and replacement of anything that's worn, or swapping the entire center section for one in better shape.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!