Really no way to reset it. If you turn the vehicle off and then back on, it will turn the light off as long as there are no faults present. As soon as the Transfer Case Control Module (TCCM) detects a fault, the Service 4WD light will come back on.
To diagnose why the message is coming up, it will need to be scanned for codes. I would suggest an inexpensive code reader or a trip to a parts store that offers "free code scans", however, the TCCM cannot be accessed by a generic OBD-II code reader. It requires a high-end scantool or the factory scantool (Tech2). This is the only way to know for sure, whats going on. 90% of the time, however, its the range position sensor in the Transfer Case (also called the Encoder Motor Sensor). The transfer case does not have to come out to replace that part, only the electric motor that hangs on the bottom.
There is also a software update for the TCCM that addresses multiple concerns, however if the sensor is bad, the software by itself won't fix it, and the software update must be done at the dealer.
Please let me know if you have additional questions!