It sounds like what happened to a lot of the Tacomas of similar vintage. Whether it was bad steel, or something else (like salt on the roads in winter), the frames just don't last. Around here, the scrapyards are full of vehicles (of all makes) with relatively low miles, that are total rust buckets with good drive trains. Rust-proofing my eye. With all the alloys, galvanized sheet metal, and whatnot, rust-proofing only clogs the drain holes designed in to prevent rust. As Neil Young sang: "...rust never sleeps...".