Unless you've spent the past half dozen years doing doctoral research among the Mud People of the Upper Amazon, you have taken note of the phenomenon called the sport-utility vehicle, the SUV to some, the sport-ute to others. You may even be an owner. If so, you are a member of one of the fastest growing clubs in the country. Virtually every manufacturer selling cars in the U.S. offers an SUV, and there are more coming. But none of them, not even Jeep, can come close to the sales champ, the Ford Explorer. One of every four sport-utilities sold is an Explorer. Last year that meant 402,663 units, and that made the Explorer the third best selling vehicle in the country, trailing only a pair of pickup trucks. With the addition of an excellent new overhead cam V6 engine this year, the Explorer's numbers are likely to get even better. The new V6, which has a single cam over each bank of cylinders, with two valves per cylinder, displaces 4.0 liters, produces 205 horsepower and 250 pound-feet of torque, delivered to the wheels via a new five-speed automatic transmission. The basic 4.0-liter overhead valve V6 and the four speed automatic are still available, as is a 5.0-liter V8. A simple three-position dial on the instrument panel controls the four-wheel-drive system. The normal mode is Auto. This mode continually monitors and adjusts power to the front wheels to minimize slip. The 4x4 High mode electronically locks the transfer case in high gear, providing a 50/50 torque split between the front and rear wheels. This mode is primarily for off road and extreme winter conditions. The 4x4 Low mode switches the transfer case to the lower gear ranges for serious off-roading. Explorer permutations are numerable. They start with a basic two-door, two-wheel-drive XL at $20,610, including destination, and stop with the four-door, 4WD Limited at $35,530. Our tester, a four-door 4WD XLT, equipped the way many are sold, sits in between the extremes at $29,635. Depending on certain option packages, the new V6 with the five-speed automatic can be added to the XLT for $425. The V8, with a trailer-towing package and four-speed automatic, tacks on an additional $1545.