The rockers seem to go. Mine have been re-done.
Sometimes it's a bad design; water and crap collect. Could be welded areas, rivots, etc.
Sometimes you get a bad batch of metal. My friend's father had the same year and model Ranger I had (01). Mine had 150k and his had 60k. His frame rotted completely and mine was fine.
Luck has a lot to do with getting a good truck that does not rot, but keeping it clean, washing out the wheel wells, frame, rockers, and anywhere else that salt, dirt, leaves, etc can collect really helps.