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One of my 923's has a similar issue. When I release the parking brake, it loses air in both systems. But as to your issue, when your air pressure drops that low, the rear brakes will automatically apply, and stop the truck. You "should" be aware of that, having a CDL with air brake endorsement...
I added a neoprene door seal between the frame and the dash. Use Armor-All (or similar) on the window seals to keep them pliable. Also added foam door seal strips at the top of the window (at the ends of the hinge) to keep water out. Works pretty well.
I think you have a combination of issues going on. The air plunger on the t-case may be corroded, not allowing the gear to engage properly. Also, DO NOT use reverse in low range!!
He does have a $hit load of old parts....and he is VERY proud of them. Sounds like he's come down on his Hummer prices though. They were in the low 20's when I was up there a couple years ago picking up a goat.
You might consider something like this (center pic). Could be used as a tool box or storage or even emergency sleeper. Lots of old truck scrap yards have them.