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I just discovered the cause of this noise on my father's 99 blazer last night. It's the rear driveshaft universal joint.
If you raise the car up at the back and slide under, you can twist the driveshaft back and forth and you will hear the clicking noise coming from the rear universal joint right where the main driveshaft connects to the rear differential. I believe the whole driveshaft needs to be replaced.
Jason