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I dont know why, I had always thought it was bad for the truck to roll on the tranny, especially if parked on an incline or decline.
FYI
This is what the Haynes Repair Manual mentions about self adjusters...
"Back the vehicle up, making repeated stops, to actuate the self-adjusters, which work only when the vehicle is in reverse."
Parking gear inside your tranny is a physical bar/gear setup. There isn't anything that's going to 'slip' once its engaged. Unless of course you have a manual tranny, then there is no parking gear and it can slip.