Ford pickups have been doing the country’s work for 66 years. They’ve hauled grain, towed logs and plowed snow. They’ve cleared debris after tornadoes and pulled floats in the Rose Bowl parade. They’ve shouldered those loads with parts forged from steel. Until now. On Monday, Ford unveils a new F-150 with a body built almost entirely out of aluminum. The lighter material shaves as much as 700 pounds off the 5,000-pound truck, a revolutionary change for a vehicle known for its heft and an industry still heavily reliant on steel. The change is Ford’s response to small-business owners’ desire for a more fuel-efficient and nimble truck — and stricter government requirements on fuel economy. And it sprang from a challenge by Ford’s CEO to move beyond the traditional design for a full-size pickup.