The driveline parking brake is on the back of the transmission.
So the rear axle could slip into neutral as it falls out of high gear. In which case the truck may roll away and the driveshaft need not turn.

So it would be wise to follow the operators manual/instructions and park the truck with axle in (usually) low gear.

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-