My 1956 Chevy vacuum Shift rear works as Willy described...in Hi rear the acutation shaft attached to the shift lever for the rear is pulled UP. With the truck OFF there is no vacuum holding the actuation shaft up. Over time with the truck parked gravity could allow the shaft to slide back down (remember these parts are 50 years old and broke in well) and right at the moment it drops from Hi and just before it goes into LO it will be in between gears. At this point you should still be OK as there will be pressure on the rear gears holding them from shifting, but if someone bumps the truck or slams the door it could allow the shift process to start, putting the truck in a freewheel condition...remember your hand brake is attached to the drive shaft ahead of the rear, so that won't help.
I was taught by the old timers to always park in LO and chock your wheels...
Mike B
