Unfortunately, that adjustment is for pre-loading the gears during assembly of the gearbox. If you change it then you can possibly put the worm in a bind or worse. You supposedly cannot return it back to zero without disassembling the box.
I got a gearbox for my 88 Bronco II for $25 at salvage, off a wrecked ranger showing 80k miles. (price was for me to remove it) I think they are all the same from like 85-92. Took about an hour to switch out, and you need a pitman arm/ball joint separator tool. Rebuilt gearboxes are like $150-ish
The slop in the steering is usually from the outer tie rod ends. Keep in mind that Bronco IIs tip over easy so I would spend the $ to keep that steering in top shape.