Most hijacked trucks are the result of teamwork between warehouse people and driver. Sometimes the driver is left out of the loop. But as a rule, the hijackers know exactly what is on the truck and where the desired item is located inside.
WalMart guns are brought by UPS, and must be signed for. Not the WalMart truck.
Random hijacking of a WalMart truck would be hilarious. Going through the Bycycles, Diapers, Bottled water, Charcoal, Ice Melt, Motor oil, etc to try to find the ammo.