Rear Suspension Clunk 99 4-Runner

I hear a loud clunk from the left rear of the vehicle, but ONLY when it's below 40 degrees and ONLY when I hit a large bump such as a railroad track. It only happens once, after that it won't clunk until the vehicle sits for a few hours. It sounds like a loose jack or tool box, but the above info tells me it's not. Thank you in advance for your advice.