When the starter goes out, it's usually just because the brushes have worn down. What's the mileage? They seem to last about 200,000 miles worth, depending how many times it's started. If it won't start, but then will start if you bang on the starter, then it's the brushes, since banging on the starter will get the springs to push just ever so slightly more on what's remaining of the contacts and get you another start.
It's possible that all you need is a brush assembly. These are the same thing for most Ford starters, but it usually isn't listed as a specific replacement for an Explorer.
I bought one at O'Reilly Auto Parts, back when they were ~$20. The price seems to have skyrocketed to $38.
http://www.oreillyauto.com/site/c/detail/BWD0/X379.oap?pt=01923&ppt=C0330
You can get generic ones on ebay for cheap, or you could call around to various parts stores and try to track one down, but it might be tough to do, since most parts stores just sell parts based on make and model, and that brush assembly isn't listed as an available part, even though it works on most Ford starters for the first gen model years, and a lot of older ones. You could also always try the dealership, Ford/Motorcraft does offer the brush holder assembly as well.
As for rebuilding, you don't even need to take the thing completely apart to replace the brushes. You can just take off the end cap, unscrew the terminal from the solenoid, take out the old brush assembly, put in the new one, replace the cap, screw back on the terminal, and be done. Of course, if you have the time and the skill, taking it apart and cleaning off all of what the old brushes left behind can be a good idea, and should make the armature and all of the other parts last a lot longer. You don't even really need a rebuild kit, unless the bushings are really worn. If they aren't just re-assemble, either using new grease on the bushings or leaving the original grease on there in the first place.
I'd definitely say the new brushes are the way to go. For ~$20 or less, you can have a like new OE-quality starter that's better than the $100+ China-made junk ones they sell these days. It's so easy to replace it doesn't make sense to do anything else unless you have more serious starter problems and really need a new one.