OSHKOSH, Wis. - At the top of a 10-foot-high mound of dirt, Gary Schmiedel takes in the silence. The military truck he's driving barely hums just before it careens down a steep incline into a muddy pool. Normally the vehicle — a Heavy Expanded Mobility Technical Truck or HEMTT — would be so loud the occupants wouldn't be able to talk to each other, said Schmiedel, vice president of product engineering for Oshkosh Truck Corp. But this version is about as loud as a standard sedan, with a smooth ride, splashy computer screens and a comfortable interior. This isn't your...