The only way to determine where your problem is would be to raise the car safely and use the 4 P's; Push, Pull, Pry and Prod on everything you can find under the rear suspension until you find the loose part.

Parts to check are the differential mounts, subframe bushings and dog bones. You can tell more about the dog bones by looking at the rubber joints. If there are pieces of rubber sticking out of the rubber joints, replace that dog bone.

Good luck.