I was going to say shock mounts, but normally they would make noise over bumps, railroad tracks, etc....could be the spring seats as well, since they utilize insulator pads.
I think you may have a torque-related issue. It's about the only thing left that makes sense, since you say it's primarily when you're pushing and releasing the gas pedal, and low-gear shifts. Virtually the whole powertrain shifts when you add or remove power from the engine, as does the rear axle. It IS possible that the shock mounts or spring seats have worn to make noise only when torque is applied, since this is also a common, rotational wear direction--whereas going over bumps and tracks is more of a vertical suspension movement. Part of the reason why BMW's are so smooth and vibration-free is because they pretty much mount the whole power and drivetrain on rubber insulated mounts, including the rear axle (diff.), transmission, etc.
If you're sure it's coming from the back, (or maybe underneath the car?) take it to a shop, and have the mechanics put it on a lift with one guy in the car. The guy in the car can start the car, put it in gear, depress the brake firmly, and simultaneously torque up the powertrain by stepping on the gas pedal a bit (This is also known as "power-braking"). That way, another guy under the car can have him repeat the cycle while looking, listening, and feeling around for your clunk.
I've figured out A LOT of mystery problems with vehicles over the years using this method.
Officer: "I see you set the table for four." Groucho Marx: "That's nothing--I set my alarm clock for eight."