Here are a few tips to look out for in a '91 - '92:
AC - VERY, very, very problematic for the '91 - '92's. They got most of the bugs worked out by the '93 model. Make sure it works and there are no leaks in the system. Run it for at least 20 minutes and monitor it. Ifit starts dumping warmish air on you, it's got problems that aren't really cheap to fix.
Power Steering - Check for leaks on the pulley shaft. It actually won't start to leak until around 100,000 miles or so. Not a biggie to replace, but you will need a special puller to remove the pulley (it's plastic). You can always take it to an AC shop to have them remove it for you.
Radius Arm Bushings - Hopefully thosehave been replaced by now. If not, it can be done, but it's a pain to remove the factory rivits that hold them on. There was an earlier recall that the factory replaced them for free, but I'm not sure if it's still in effect. Check to see if they've been replaced.
Manual trannys need to have the rubber shift rail plugs replaced by no later than 100,000. They have a tendancy to rot and tranny oil could spew out the back toasting gears... not a pretty sound and very pricey.
Rear Hatch Struts - Watch them about 90,000 or so; depending on use. Some fail slowly, some just fail. A rear hatch on the head will get one's attention really fast.
Gas Filler Neck - I think there was a recall as well on this. Anyway, I had mine in for something that the dealer dealt with. He also took care of the rear hatch strut issue at the same time.
Sun Roof - Another recall item. If you have a removable sun roof, and you do not reinstall it correctly' it could fly off. Big problems for the guy behind you.. look into that too.
Alternator will suddenly die about 80,000 miles. Some sooner... some later... but it will die. It's actually one of the few things in an Explorer that is easy to replace without have to tear half the engine apart. It's about a $130 item by doing it yourself.
Hubs - Just a plain stoopid design...period. If they don't lock or are making a racket when you put it into 4X4, the plastic locking mechanism is broken. Save yourself a lot f grief and money by installing manual hubs. Contrary to popular belief, you can leave your hubs locked in. You only have 4 wheel drive when you lock in the transfer case. At worst you'll lose about 1 mpg, if that by leaving the hubs locked. The replacement plastic parts are actually more expensive than manual hubs in the long run and will probably break again anyway.
4X4 Push button - It's a crap shoot if works at that mileage. The motor that activate the transfer case is really particular and a pain to deal with. Why couldn't Ford just use a manual lever and keep it plain and simple.. sigh. I had mine lock into 4X4 and I had to take it to the dealer to get it out. He struggled with it for most of the day until he got it to disengage. 4X4 Low is also a problem. Sometimes just won't work and when it does, you sometimes can't get it out of low. If you aren't already doing it, try putting it into 4X4 mode at least once a week just to keep things functioning...
Ivan
