I go along with macx,I ran synthetic oil in my rear end,about a couple days later I got the same thing.It almost sounds like a popping/creeking sound,only in turns and I found out it was due to a lack of friction motifier in the rear.I changed back to reg 90wt with the friction modifier and the problem was gone.Clutches in the diff were hanging up.Dont know if the modifier is broke down but you never know.
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