Why won't your 1992 Explorer go into 4-wheel drive even though the indicater light shows it's on?

Have a 94 ford XLT, check you control module first, it is located (94 explorer ) driver side rear behind the inside panel( the whole cover where your seat belt attaches and jack storage) there are screw under the carpet and the push lock removed, and the seat belt bolt it is a star, got one at auto parts for 5$ just pull your cover off the driver side seat belt on the top (inside) check for size at store before you buy.( the one the said, didn't fit took a couple out checked it, bought the one that fit, it will work on the back also) Any how there is a box in there it is your 4x4 computer. there is a button a light on the bottom, you will have to look up the flashes i do not remember. You push the button if it stay on, or off replace, I think it is supposed to flash like 3-4 times then go out. If it flashes continually it is bad. there are forums on how to clean and repair the computer, (mine had a broke transistor, soldered it on fixed the dash lights, bought a solder kit 10$ harbor freight with solder. Did whole thing in under a hr. If control module good, then check computer, you can also go to junk yard and get a couple of the dash switches (but what if they are bad, I would get two)swap them out see if that fixes it. hope this helps I always see the ford 91-94 for parts. So I am assuming that this information will help.