Thanks to this site and all your posts, I fixed mine in 5 minutes for $200 plus tax. I had the same symptoms. 4x4 was working for 10 years, then on the day I went to sell it, it failed on the test drive! The automatic function stopped working a few years ago and was thought to be the module at that time, but mechanic said just keep pushing the 4-high button manually when needed and save the cash.

Here are some helpful tips in hind sight on diagnosing the module as the issue:
1. Check the fuses, both under the dash and in the engine compartment. There are about 5 or 6 of them and they are listed in the owners manual - look for all that reference "4x4".
2. When starting the engine, watch the diagnostic dash lights. In my case, I only saw the "4x Low" light up when the key was first turned. on. After replacing the module, BOTH the 4high and 4low lights came on during power up which seem to indicate the 4-low part of the module was responding but the 4-high was faulty.
3. Knock on the transfer case motor with a wooden mallet or gently with a hammer while the engine is running, in park, and someone is pushing the Auto and 4-H buttons. The motor can get a dead spot or bit of dirt. The tapping can knock it loose according to the mechanic at my dealer.
4. Pull and re-seat all cable connections to the transfer motor, 4x4 module, and maybe even the dash switch.

If all these fail, the next most likely cause, especially if the 4x4 was working one day and not at all the next, is the 4x4 control module. This came straight from the ford dealer mechanic's mouth.

NOTE: Just to be safe, disconnect the black negative cable on the battery during the swap out of the module to avoid power spikes, which are known to damage them. As others have said, it took literally 5 minutes to replace the module on the 2002 Explorer. Do NOT yet remove the 2 screws attaching the plastic to the bracket. Remove the single screw that points straight up and attaches the bracket to the AC. The whole bracket and module will swing to the left and pull out once you disconnect the plugs. 2 screws hold the module to the bracket.

I got my part directly from the dealer for $200, but their list price was $280. I simply told them I was just about to sell the car and it failed. I also showed them a printout from rockauto's site showing the same Motorcraft part for $170. The part's guy said $170 was hard to believe - that was near cost. I asked how low he could go. Could he give it to me for $200? He calculated a second and said, "how about $202?" SOLD!

Lesson learned: Always ask for a lower price, especially on high-markup dealer parts. Come prepared with proof of an online price from a reputable supplier. Worst they can do is say no. At 70% markup on this part, they had plenty of room to come down a bit and still make money.

The happy ending: Took 5 minutes and a couple of small metric nut drivers to replace the module. Reconnected the battery and the 4x4 was in working order. Sold the truck three hours later. WooHoo!

If this still had not fixed it, the only thing left would have been a replacement of the transfer case motor. Lots of good video on youtube on how to do this. New part from the dealer was well over $300 list!!!! Junkyard had them on the shelf used for $70. Cheap potential fix. A little hard to get to without a lift but just a few bolts and maybe an hour's worth of work tops. Thankfully the module took care of it. Just make sure they cross the part numbers correctly or at least let the dealer verify the current Ford part number needed based on the VIN. Then shop the web or talk the dealer down in price. Either way it's not a returnable part so might as well find the cheapest source.