One problem related to axle hubs has been reported for the 1999 Ford Explorer. The most recently reported issues are listed below.
While driving at 70mph, flat, smooth road, I felt and heard a "thud" from left front area of vehicle, followed by high pitched squeel, pulled off road and did a visual check & found nothing out of place, or any loose or missing pieces. Pulled back onto highway, slowly increasing speed, everything seemed fine. After 5 to 6 miles I had to apply breaks hard, vehicle pulled abruptly to the left as I pulled back to right it pulled very hard to right w/ a grinding noise, had to slow to 30 mph to maintain control until reaching a service facility. Upon inspection the mechanic found the left front hub bearing had failed, he explained it would have become uncontrolable if I continued to drive it. Hub bearing was replaced with a new unit, old part is still available, I have sealed in box replacement came in.