1993 3/4 4X4 Suburban

I have a 93 suburban 3/4 tin 4X4, when going uphill at around 65 -70 miles per hour, I hear an intermittant clunk from the drive train. One mechanic said it was the transfer case, although it wasnt in 4 wheel drive.
it appears to happen only under load going uphill and only after driving for a while more than an hour.
any clues?
thx Steve
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Maybe a tooth busted off the ring gear?

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