I read this on a toyota parts website about 3rd Gen 4Runners:
Frame Rust: Many third-gen 4Runner owners have experienced issues with unusual levels of frame/undercarriage rust, which can lead to suspension system failure. This problem shouldn’t be attributed to the SUV’s age but rather the way its frame is constructed, which allows moisture to enter inside the frame and rust it out. A frame replacement is a good way to fix it.
I myself didn't get spend much time inspecting the frame before I bought mine. Luckily, mine isn't as bad as the poster's, but I've had to pay attention to it. I don't think Toyota issued a recall on this problem. Instead they advised owners to clean their under carriages after exposed to salt.
The 4runners do have rust. But no where near the issue that Tacoma’s have. So that’s why there hasn’t been a recall.