Have you been able to visually watch the air high/low shifter work? Sounds as though you might be getting 1-4 and not actually shifting to the 5-8.
Chas
Chas
I'm guessing that this is the forum that you were referencing @tkv000If it's left hand drive, I posted the measurements and adjustments a while back for the linkage, I'd start there.
Thanks @Iain_Unimog. That should come in handy. I noticed it had some of the parts for the air system as well. When it gets cold here, it seems to freeze something in the system and not want to build pressure past 7 bar (mine is a 20 bar system). So this should be handy in this respect as well.From your description it sounds like you need synchronizers for at least first and second gear. Unfortunately your pictures don't show the synchro, just the main gears.
For the section you are tackling I would look at leaving the transfer case and axles alone and just pull the 4 speed main transmission. That will cut your weight down a lot, and get you access to what you need. If you pull the entire thing together splitting off the 4 speed will be one of the first things you need to do anyhow.
You may have already mentioned it, but it's possible the hard shifting is oil related. With heavy oil in mine it was almost impossible to shift. The lighter oil made a noticeable difference in the shifting. I don't believe that will have any effect on popping out of gear though.