There's a nut on the other side of the door jamb sheetmetal. It sounds like the sheetmetal on your jamb is broken, yes? You'll need to cut a piece from a scrap cab and weld it in place; try to make a template from surrounding reference points on the scrap cab before you cut, so you can position it as close as possible on your cab.
Then replace the striker bolt and nut with either brand new ones, or ones from the donor scrap cab. You'll need to remove the B pillar plastic trim to reach inside the cab to access the nut, which will fall down loose once you remove the striker bolt. Some care must be taken to pull the trim plastics - to get the B pillar trim out, you must first remove the lower rear window trim, which first pulls out at the bottom, hinge it toward you a slight amount, then push up. If you have the jack storage tub behind the seat it will need to come out, too. Then remove the shoulder belt retractor, screws out of the B pillar, hinge the B pillar plastic inwards at the bottom and carefully slip it out of the upper rear window trim - it has tabs that interlock with that trim.
They definitely take a beating when the hinge bushings are let go for too long:
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Richard