The only Toyota frames with corrosion issues due to poor manufacturing, were those produced by Dana Corp for Toyota. Toyota replaced a lot of frames, or bought vehicles back. They also won a large lawsuit against Dana Corp.
The 4Runner never had a frame made by Dana Corp, as 4Runners have always been made in Japan. So it was not part of that frame replacement/ buyback campaign.
Now anything can rust or corrode in a highly corrosive environment, if sufficiently neglected.
Every owners manual I've ever owned specified cleaning off Salt etc. regularly, when used in a highly corrosive environment.
If you had rust issues, it's because you neglected to clean it often enough.
Having worked on all makes and models where neglected vehicles eventually rust/corrode away, I can say that all brands are susceptible to owner neglect.
I doubt Toyota has done anything different to protect further from rust. So I'd suggest that whatever you buy, be more proactive in keeping the rust at bay.
Incidentally, I owned a 3rd gen 4Runner for 22+ years from new. It was operated on Salt encrusted roads all winter long, and on the Bonneville Salt Flats every year for Speedweek.
When I sold it it had no rust, as I washed it frequently (particularly the undercarriage) whenever it had been operated in that highly corrosive environment. So that could have been several times per week during winter.
People that live in the rust belt either take steps to keep rust at bay, or the cost of having to replace a vehicle sooner than later, becomes the cost of living where we do, and/or the choices we make.
