Its probably just a U-joint that is bad.
Drive line noise is sometimes hard to diagnose, because it can transmit through the drive shaft and sound like its coming from any where , especially when your sitting in the cabin.
It could also be a bad suspension component , such as a bad spring, or control arm bushing. When torque is applied to the rear end it wants to rotate upward, the upper control arms are there to absorb this force. If a bushing is bad it may make a noise as torque is applied.
I suggest you put the rear up on jack stands , and get under the car and look for any slop in the drive line. It helps to have a helper that can rotate the wheels while you listen for the noise from under the car. If its the U-joint it should be pretty apparent when you turn the drive shaft. Check the pinion flange for side to side or up and down lateral movement as well. Also check the tail housing bushing on the transmission for any movement by pushing the drive shaft up and down .
A suspension noise may be a little harder to diagnose, because you cant put the force on it that the engine can.
If you are not comfortable under a car and are not sure of how to go about diagnosing the issue then I suggest you take it to a reputable mechanic or someone who is a bit more confident in doing the job.