He served in World War II as a SeaBee on the Pacific island of Peleliu a few arriving a few months after the Battle of Peleliu. In the image my grandfather is engaging in some hijinks with fellow SeaBees, and they were trying to get the military Jeep to go airborne on a small hill on the rocky beaches of Peleliu.
On an island with no women, scarce alcohol, and little in the way of entertainment (not to mention encountering the occasional solitary Japanese soldier who had refused to surrender), you take advantage of any opportunity for amusement that presents itself.
My grandfather's primary role was as a vehicle mechanic, so I suppose he could justify such tomfoolery as a "road test" of a vehicle sent in for repairs. He also has some funny stories about "borrowing" a steamroller that he fixed to tool around the island with his buddies.