i'm about 99.9% sure it's the shift motor. 2 out of my 3 Explorers have had bad ones. They're not that hard to rebuild, but extremely expensive to take to a shop and have replaced. somewhere over on explorerforum, there's a good thread on how to rebuilt them. If you want a super easy temporary way to get 4wd if you MUST have it, take off the shift motor and the little gear wheel that it turns on the t-case (it's held on by those bolts with star shaped heads) and use vice grips to turn the shaft. You'll hear a clunk noise when it engages. IIRC, turn clockwise and the first clunk is 4hi, second is 4lo. of course, this isn't a good thing to do when you're up to your hubs in mud, but it's an option if you truly NEED 4wd.