Ford 4x4 2002 explorer will not go into 4 wheel drive

First off is it the shift on the fly 4x4, if it is you can try this, don't ask me how I know. On the transfer case, drivers side is an electronic servo motor that shifts the unit from 4 hi or low, mine used to not work at times, I took a rubber mallet and gave it a couple of whacks and guess what it worked again, your out nothing but a few minutes of time to see if it works.