1998 Ford Explorer ABS sensor

The person that told you that was wrong on two accounts.
1. It's not the rotor that the speed sensor is part of, but the wheel hub. A new one that will last you a few years can run up to about $225 for the part if you can change it youself. Get a repair manuel when you buy the part if you go that route.
2. You can change the sensor itself, but they run about $160 and you can only get them from a Ford dealler. This is a much easier repair, but you may find out latter that the hub needed replaced also. You should get the hub checked by a mechanic before you do this repair. :)