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If it happens only (or is much worse) in 4wd, then I'd bet money it's one or both CV axles. Look at the boots over the joints. If any of them are torn or have holes, that's most likely your problem.
I had the same kind of noise comming from one of my old cars and everyone said to check the CV, and sure enough, the CV was toast and making the noise when i turned. So that is what i would put my money on.