A very common problem, and best place to start is to look at the exhaust on the rear where it goes over the rear suspension. Often times the exhaust hits the linkage back there. It causes your complaint exactly. The fix is to loosen the two exhaust bolts, and rotate the exhaust slightly.
The axles can also make noise. If you shift from drive to reverse tou might hear some clunking. If it's the diff you will an obvious knock from the back. Look at the exhaust first, and go from there.
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