The company Ford insists on installing in Explorer just factory produced lamps. Vehicles of the brand of 1998 year use exactly 12 V voltage for these light bulbs. For every vehicle including Ford Explorer marker lights are very important. You won’t get normal work of markers in Ford Explorer 4.0 with other types of lamps. But usually you can install W2.1x9.5D base bulb of any brand providing it has power at 5 W.
So, make sure you put into 2001 Ford Explorer high quality and appropriate marker lights. It’s usually hard for Ford’s cars to install lamps with no skills, especially in 1997 Explorer. It also can be difficult to buy W2.1x9.5D bulb with 5 W capacity and just 12 V of voltage.