I've been having this noise comming from under my 2000 4Runner for a couple weeks now. Whenever I hit a bump, pot hole, or anything that causes the truck to shift from left to right it sounds like there is something loose underneath. I can hear it and feel it. Yesterday I finally removed everything from inside the truck that could possibly move, dropped the spare from underneath and crawled under. Everything that could possibly become loose or be removed is bolted down tight. Since I don't notice anything going over speed bumps when both rear wheels go over at the same time only when they go over bumps independantly I started to shake the truck. When shaking it side to side it sounds like there is something inside the rear axle that is shifting back and forth. It makes the same noise I hear inside the truck when I hit bumps and I can feel it if I place one hand on the backing plate. The axle is stock 4.10, open diff. and never been opened or disassembled for any reason. Any idea what could be causing this?