Ford Repair: 2001 Ford Expedition 4x4 Indicator Light, vacuum leak, proper diagnosis

Question
No codes come up on the computer, and the owner's manual is the only indicator that the system needs to be serviced.  My mechanic thinks it's an acctuator, because he checked for a vacuum leak, and also suggested it might be the switch.  
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Followup To

Question -
I have a 2001 ford expedition that recently the 4x4 light began flashing.  The manual says "service system", but does not indicate anything in specific.  Any suggestions?

Answer -
First step is to find out what codes are stored in which modules.

Answer
So did your mechanic check all the modules, or just the engine computer? The transfer case is controlled by the GEM module, that is why I suggested checking for codes in all modules. These modern vehicles are so multi-plexed the codes have to be searched in each module because something we don't think is related may lead to the proper diagnosis.

The PCM does not directly control the transfer case, so it probably won't have codes related to that system.