Crawl underneath and look. Areas of concern would be just ahead of the rear axle where the floor transitions to cargo area. It's kind of in line with the front of the rear wheel wells. There are a bunch of panel joints there, and if the tub is starting to rust, it well start there.
Here is an example from my Jeep. See how the rust is around the edges, but not so much in the middle of the panels? Those are the seams and the rust starts there due to the welding. Welding Burns the galvanizing away and the seams are open enough to allow water and salt in. They start to rust, and it's a downhill slide. If your frame is rusted through, and the tub is going, kick the project to the curb. If the tub is solid, them it might be worth it.
I have the opposite problem... Good frame, bad tub...
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