As Tim notes above, vacuum shift rear ends tend to shift or "default" to low range when the truck is off and vacuum is lost. The danger of this is when your truck has a drive-line parking brake with the drum just behind the transmission. When the vacuum bleeds down and the rear end drops out of high and before it engages low, you are not in any gear. Thus, the truck could roll if on an incline. That is why you will read and hear about leaving it parked in the low range. My COE has parking brake cables to the shoes in the rear, so it is not a problem. My Diamond T has a two speed rear end and drive-line parking brake and the manual instructs you to leave in the low range.
Kent
