The most common reason that the 4x4 mode is not working correctly for Your 2002 Ford Explorer Sport Trac is going to be due to a failure of the actuator within the transfer case. This will stop You from being able to enter 4x4 mode and instead You're going to be in Your normal gear. A replacement for this part is not expensive with more stores having one for between $30 and $50. The issue here is that it takes a lot of work to get in to Your vehicle and repair this so if You want to take it in to a shop the total cost will be between $300 and $600.
Gregory N.
April 9, 2020
Chances are there is a transfer case problem with the actuator. When someone tries to engage the 4wd, test to see if there is control. Disable the actuator if there is control, and have it tested.
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