knocking sound from rear axel

I have an 87' chevy 1/2 ton pick-up and when i decelerate i hear a rythmic knocking sound coming from the rear axel doesn't happen all the time. but it's getting worst.. any suggestions would be welcome

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First, stop driving it. Second, pop the differential cover and have a peek. You're looking for broken ring or pinion teeth, broken teeth in the carreer gears, or metal chunks/flakes/shards in the fluid.
Doc

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the pinion pin is probably rounding out betwen the helical gears but you will save the ring gear and pinion gear.
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