I have a 2000 which I purchased in about 2005-2007--somewhere in there. Within the past five years or so, it has developed an issue that I notice accutely if I get to 70 mph or above. That issue is that if I hit any kind of bump, or anything out of the ordinary in the highway, it will start rocking back and forth, left to right. This seems more pronounced the more people or heavier objects are in the vehicle. It went for years without doing that, so it would be hard for me to say that it's something inherent in the design of the vehicle. I upgraded the rear springs several years ago (I don't remember what the upgraded ones are any more), and I changed the shocks to some Bilstein shocks (don't know the model as my mechanic did it). The rocking seemed to go away after I changed the shocks, but then has returned with a vengeance. And I may not have had a big enough load inside during testing to notice it as much. Any thoughts on this?

I can almost guarantee it’s your rear lower control arm bushings. I had the exact issue around the same speed when hitting a bump in the road and it would sway left to right like a pre-teen girl at a Justin Bieber concert.

Mine were beyond toast and the problem went away when I replaced the control arms with some upgraded aftermarket ones.

You might be able to see the condition of the rubber without unbolting them if you get underneath with a flash light, but visible cracks in the rubber or not, it’s those bushings I’m sure.

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If it happens at low speeds as well or after sharp turns it's worth replacing all of your sway bar bushing and sway bar end link bushings. Their primary purpose is to reduce body roll and made a massive difference for me when I did.

I have the rear end links on my radar to do in the near future as well as sometimes the rear end of my 4Runner will feel like it "skips" over large bumps or potholes. The front end is rock solid but the rear end moves and then quickly corrects.