For many years, General Motors has had the full-size four-door sport-utility market to itself. The Chevrolet/GMC Suburban, augmented in 1994 by the slightly smaller Chevy Tahoe and GMC Yukon, were really the only choices for those who wanted a lots of interior space, heavy-duty towing capacity and, properly equipped, the capability of taking to the backwoods. The only other entry with similar size and capabilities was the warlike AM General Hummer. The picture has changed now. With its new Expedition--soon to be joined by a luxury Lincoln version called Navigator--Ford has launched a serious bid for a share in a small (but growing, and lucrative) market. The price of size and V8 muscle, of course, is indifferent fuel economy, even in a class of vehicles known collectively for thirst. And with the Suburban, at least, garageability can be an issue. But with the world's lowest fuel prices, the mpg issue is offset for U.S. buyers by the appeal of generous interior volume, comfort and the security of being surrounded by lots of structure. These are, obviously, big vehicles for big jobs. And unless you're planning to tow a bulldozer, the Expedition stacks up as the best of the bigs.