If you end up driving the truck in High rear all the time, put it in Low rear when parked. The shift linkage can fall out of high gear due to gravity and the rear can end up in neutral allowing the truck to roll free.
Mike B

Can you explain that a little further. This is my first vacuum 2 speed and I haven't even driven it yet so I don't know what I'm talking about...
My understanding is - no vacuum no go. It takes vacuum to hold it into gear whether it's in Hi or Lo.
So, doesn't the rear end go to neutral no matter where you put the selector switch, without vacuum?
Thanks,
Greg