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?

Could be one of many things. Sway bar endlinks or brackets, panhard bar bushing, control arm bushings, shock mount, etc. The only way to tell is to jack it up and start pulling and twisting on things, and doing a visual inspection.

I spent an hour under the truck yesterday. Checking everything from front to rear. All the things you mentioned are bolted down tight. Rear shocks, rear swaybar ends links and the brackets mounted to the axle, lower control arms, upper control arms, and panhard rod (an adjustable one from Sonoran Steel). Nothing is loose. I couldn't do anything to make noises that resemble what I hear until I sat under where the spare goes, grabbed the frame and began shaking it side to side. Immediately it starts with the noise. I can feel it when placing one hand on the either of the backing plates and shaking the truck with the other. It certainly seems to be coming from the axle.

Thats weird. I don't know what could be loose inside of the axle. Maybe a spring or something is loose in the drum? i guess you could start by pulling the drums.

I would jack the rear wheels off the ground and then push and pull on the wheels to see if there is any endplay in the bearings. If getting both up at a time is difficult do one side at a time.

Thanks for the repsonses guys. Tomorrow I'll jack up the rear end. I can get both ends in the air at the same time on jack stands if I take the wheels off. Then maybe I'll figure something out because I really can't believe there's something in the axle causing this but I sure can't see or feel by hand anything causing this.

Could be a U-joint, especially if they are still original.