The specific fuse number and location will vary slightly depending on options and trim level. You'll need to consult your owner's manual to find the correct fuse for the fuel pump relay. The manual will have a diagram showing the fuse box layout and the function of each fuse.
If you don't have the owner's manual, you can often find a digital version online by searching for "[Year] [Make] [Model] Owner's Manual". Alternatively, a search for "1998 Ford Explorer fuel pump relay fuse location" may yield helpful results from forums or repair websites.