I'm pleased to see it's all back together and you can do trips in it.
After a transfer box rebuild a couple of years ago I had a problem with it popping out of gear. It turned out to be the foam pad that goes in the opening in the transmission tunnel. The mechanism was so free and easy after I'd put it all back together that the pressure of the foam was sufficient to push it out of gear, especially with a bit of torque, such as when going uphill or pulling away from a junction. I hope it's something as simple in your case. You should be able to feel the detent balls latching it in place in low or high. They're not very stiff to overcome but there's a distinct notch. Inside the box there's just a splined slider that engages one gear set or another, so there's nothing in there that would tend to push it out of gear. Anyway, hope you get it sorted.
After a transfer box rebuild a couple of years ago I had a problem with it popping out of gear. It turned out to be the foam pad that goes in the opening in the transmission tunnel. The mechanism was so free and easy after I'd put it all back together that the pressure of the foam was sufficient to push it out of gear, especially with a bit of torque, such as when going uphill or pulling away from a junction. I hope it's something as simple in your case. You should be able to feel the detent balls latching it in place in low or high. They're not very stiff to overcome but there's a distinct notch. Inside the box there's just a splined slider that engages one gear set or another, so there's nothing in there that would tend to push it out of gear. Anyway, hope you get it sorted.