This happens all the freakin' time. Cheap steel from China. It's a common problem with Rangers from the 90's.
(Anyone reading this who owns a Ranger from the 90's, but hasn't had this problem, keep an eye on your leaf spring hangar brackets. If I had one from that era I'd take 'em off and clean 'em up and coat them with undercoat stuff.)
Your best bet is to mail order replacements from a junkyard down south where they don't use salt on the roads.
Up here in the rust belt, those spring hangers are scarce in the junkyard. Plus if you do find some, they're not in good shape.
I know two guys with 90's Rangers who have had that problem. Both of them ended up with the leaf spring poking up through the sheet metal bed because they continued to drive it like that. (Idiots, I tells ya!)
It looks like yours isn't poking through the bed yet, that's great. DON'T DRIVE IT til you fix it. In fact, get that rear end up on jack-stands ASAP, til the tires almost come off the ground. Jack it up by the frame, not the axle.
Replace all 4, and wire brush and get some really good under-coating stuff to put on the replacements before you install them. And do what Weezl said; use either grade 8 bolts or better yet, get some grade 9 aircraft quality bolts from your nearest municipal airport with a shop. They ain't cheap but that's what I do when I need a really high quality bolt. (The wings on my ultralight airplane are held on by only four tiny little quarter-inch bolts, two at the fuselage, and one at each end of the struts. They're grade 9 and the plane is rated at 6 positive G's. Good ****!) And change the bushings while you're at it.