If I let off the gas and the car is above 2000 rpms there is a contact like popping noise in the rear, sounds and ffeels like the rear end is hitting the bottom of the car. I had a shop look at it when the noise was not that bad but they could not get the noise to happen again, and they said the rear assembly was fine and the suspension looked all attached fine and in place.
the noise is getting worse and I got under the car myself and everything looks normal, all upper and lowers look attached, and I did not feel any play.
Check the upper and lower shock bushings, they can make a nice knock or popping sound. The pinion snubber usually doesn't pop or knock, because it's made of rubber. Upper control arm bushings are also suspect, and it's hard to diagnose them in the car. You can use a flashlight and see the bushings, but it's hard to tell if they're bad or not. Mine were easy to diagnose when they were shot, I could see that the bolt wasn't centered in the hole anymore, and could see the tears in the bushings. Typically, you'll just see some stress in them, and they'll look a bit dry-rotted.
Can you force it to make the noise, or does it happen on certain turns, bumps or road conditions?
Torque boxes can also clunk when they're giving out, but typically they're creak and groan for a long time before they start to clunk. The problem with troubleshooting suspensions is that with it on the lift or jack, you can't exert the same amount of force that driving does, so it's hard to just grab and shake things around.
As an example of how difficult seeing the problem can be, here was my clunk in the front end, but before I took the lower control arm off, they looked perfectly normal.
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