When I bought my '92 this is what I checked. I did it myself but you can have a mechanic check for a reasonable price:
Thorough test drive - in 2wd and 4 wd
In 4wd in a gravel parking lot drive in a tight circle in each direction, if it hops in the turn the 4x4 works well. Test in 4x4 high and 4x4 low.
Drive bumpy back roads to feel handling
Drive stop and go traffic
Drive highway and feel for wander and do some rapid decelerations from speed to feel for any kind of shake or pull in the steeringthat may indicate bad rotors or alignment
Check tire tread and condition
Open the hood and check general cleanliness - lots of oil indicates a probable leak. If it's much cleaner than the rest of the truck, he may have cleaned it to hide hints of a leak.
check all fluids for level and cleanliness (dirty fluids mean lack of maintenance)
jack up front end and try to move front tires in both a vertical and horizontal plane to check the tie rods and ball joints
while jacked up, get underneath (with stands in place) to check body mounts, frame and body for corrosion
put on parking brake and put into drive to test parking brake
check all lights, horn, accessories, spare tire, jack, etc
ask for maintenance records for last few years (especially for tranny rebuild recipt if it was done)
do a carfax report
go on a pilgrimige to Mecca and pray that you get a good ride (not that the Explorer is anything but a great ride, but sometimes you can be just unlucky)
If some things don't check out from my list, it may not mean you shouldn't buy it, but will at least give you bargaining tools.
Good luck, hope it works out with you and this X they are a great vehicle and lots of fun.
