I didn’t have time to address this problem on my web site (link removed) , yet. In my opinion it is a factory mistake or better yet intentional „mistake“ of the ZF. In the back of the gear box is the reducer (hi and low change), the sleeve that changes it is manipulated by brass „pads“, the pads and the sleeve have to big tolerance between them. With time the brass wears out and the tolerance is so big you that can’t switch any more. Then you need to weld on the brass and the turner makes a lesser tolerance and you put it back in. All you need to buy is a set of gaskets. Only the back of the gear box needs to be opened the gear box can stay on the truck. There are other things that can create similar problems, but in your case it is probably it.