Look at the rear shackles and hangers, the frames dont typically rot but the shackles and hangers do. They are cheap and very common to replace so if they are bad and the frame is good there is no problem. If i were buying from a dealer i would negotiate that in the sale if possible/applicable.
Another rust spot to check out is above the muffler. Most rangers have this dumb fabric stick on heat shield that traps dirt and moisture and can cause the bed floor/supports to rot. This is a bigger deal than shackles in my mind.