Here is what I did to fix mine.
On the motor there is a cover that has wires going into it.
Take it off and remove the gear.
The gear has screw on the back side that is used for a "stop" to keep the motor from turing too far.
The screw has a plastic ring around it that breaks and lets the motor go past the stopping point.
I replaced the plastic with a small piece of vacuum hose and now the 4x4 shifts and works fine.
Here is the thread where I got the info:
http://www.explorerforum.com/forums/showthread.php?p=1102904