Engine won't start. Makes a loud grinding or whining sound instead

Well, the Explorer is eighteen years old. When you turn the key and hear a loud whining sound, that's the starter motor spinning without moving the teeth on the flex plate or flywheel. That usually means that the Bendix drive is shot or the mating teeth are worn off. A grinding sound usually means that the Bendix is engaged and grinding on what's left on the driven plate. It's likely a bad Bendix drive on the starter, or maybe broken teeth where it's engaging. See if there's plate underneath the bell housing that you can remove to see the starter drive. Use a big screwdriver to move the flex plate to get on another gear spot and see if that allows the Bendix to engage. Or just have a real mechanic fix whichever is the problem.

Sounds like a starter, however as it is grinding you may have a bad spot in the ring gear, if that is the case then the engine or transmission will have to be removed, and the flex plate replaced.