You have two items that may be worn, one is the steering sector gear, the other is the ball nut. The contact points of the two parts is worn to the point where adjusting no longer effects them. The ball nut can be turned around to make a little difference, but the sector is done for if worn. Look on E-bay or some of the venders sites that sell NOS parts for replacements. Once you change these out, your box will work as normal. Rough use and lack of oil wears them out.
I would pull it apart for a good cleaning and inspection. The recirculating balls and track also go bad, and once worn the shaft is non repairable. Bearings are another wear item, they can be replaced with new, but the lower bearing race is machined into the steering worm shaft, at least the older ones are, and is non-serviceable. Bushings are easy to get and replace, but they only hold the sector gear inline and have little to do with adjustment unless they are completely shot or missing.
Look at the gear contact points, if they have deep wear marks, you found your problem. If you look in the old parts manuals, they list a shim kit for the sector adjusting screw, this is no longer available, but it might help your situations. The shims removed the play in the adjusting screw slot once it gets some wear. By removing the slop, the sector can be adjusted tighter, but the shims were only a few thousands thick.
Joe