Have Ford Questions? Ask a Mechanic for Answers ASAP
Since you have checked all the external possibilities the only thing left is to get the Transfer Case control module scanned for codes. Just like the engine control unit, it stores codes when the light comes on and will point to the defective part. Most decent repair shops should have a scanner that can access that module. But the inexpensive code readers like the auto parts stores use will not be able to pull the code.