help diagnosing noise in rear

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.