There seems to be an unending stream of sport-utilities hitting showrooms. But none have come close to the popularity of Ford's Explorer. It has been so successful that there has been little inducement to change it. However, there are some improvements for 1998. A programming change in the four-wheel-drive system sends more power to the front wheels under hard acceleration to lessen the likelihood of the rear wheels slipping. The standard ABS has been massaged to improve control on slippery surfaces, with reduced noise and vibration when activated. The rack-and-pinion steering system has been refined for better on-center feel and quicker response. All models now have Ford's SecuriLock security system, which the company claims is virtually impossible to defeat. 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 driving off road or in extreme winter conditions. The 4x4 Low mode switches the transfer case to the lower gear ranges for serious off-roading. The Explorer offers an engine for everyone. Choices start with a 4.0-liter overhead-valve V6 rated at 160 horsepower. Next up is a 4.0-liter single overhead-cam V6 with 205 hp. And finally, the big 5.0-liter V8 makes 215 hp. Explorer permutations are numerable. They start with a basic two-door, two-wheel-drive XL at $20,405 (including destination), and stop with the four-door, 4WD Limited at more than $35,000. Our tester, a four-door XLT 4x4, equipped the way many are sold, sits in between the extremes at $29,585. That includes the 4.0-liter sohc V6, our recommendation, for $540. The V8, with a trailer-towing package and four-speed automatic, tacks on another $1600. All told, in ascending order of trim and price, there is Explorer XL, Sport, XLT, Eddie Bauer and Limited. Deciding to buy an Explorer is not too difficult. Deciding which Explorer with which engine and which transmission could take an afternoon of brochure browsing.