The jeep, the military's World War II workhorse, is about to go the way of the Stanley Steamer. The Army says it can't keep pace with modern technology. The replacement is a larger vehicle called the Hummer - the name is somehow derived from the initials for High-Mobility Multipurpose Wheeled Vehicle. The word jeep itself was derived from the initials GP, standing for the military's General Purpose Vehicle. The Hummer will be built by the AM General Corporation of Detroit, an American Motors subsidiary, under a $1.2 billion contract awarded by the Pentagon this week.