Transfer case housing. This sound exactly like what mine was doing when the housing had a hole worn in it. There is a spring that comes lose in there that gets underneath the shaft. After continuous revolutions, the shaft causes the spring to wear a tiny hole in the bottom of the t-case housing. Look under your truck and see if you can see where fluid has been spraying around under there.
Another thing to check maybe is your tires. See if you can see a bolt sticking out of them anywhere. You may not hear it at high speeds, and it won't do it at idle.
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