I was driving my 06' E down the beach yesterday , and it started knocking bad from the back. I looked under from the back while my fiend gave it some gas na dthe differential shook madly. It sounds like a sledge hammer beating on the frame, when the front wheels slip. Its a manual tranny, we toed it back to pavement, and it drives fine, sept for an occasional slight knocking. I actually drove it 180 miles back home , just the occasional light knock. Any ideas what happened?