ok so, if your explorer is a true AWD, then their is no way it would be kicking in and out, as it is a fulltime 4x4,
what im guessing you have is an AUTO 4x4, which is close to the same, most of the time it is in 2wd but when it detects wheel slippage it automaticly locks into 4wd high.
Does your x have buttons for Auto 4x4, 4x4 high and 4x4 low?
when mine was doing the same thing it was because my tires needed to be rotated. these explorers are very picky when it comes to that. its because the tires wear down and the explorer thinks it is tire slippage. also make sure they are inflated to the proper PSI (35 rear, 30 front) as this can cause it too. hope that hleps