Mine does exactly the same thing. Sounds/feels like something is about to fall out of the rear end (Like you said - bouncing around.). I am convinced it is due to clutch chatter combined with a worn trans. I have a lightweight flywheel and a puck-style friction disk. Neither the trans nor the diff are original, so mileage unknown. I have removed the exhaust, the one-piece driveshaft, and the complete rear suspension. I opened-up and inspected the diff, and inspected the axles and wheel bearings. Everything looks fine. I know that my trans is worn because of various gear-type noises that it makes. My plans now are to get a full-face friction disk and overhaul or replace my transmission. How long have you had the FE disk?