C0310 indicates a circuit problem for the encoder motor, (mounted on the transfer case). Here's the link to the diagnostic flow chart:
http://www.justanswer.com/uploads/MC...0310-Debug.pdf
C0374 is "General System Malfunction" which was most likely the second DTC you retrieved. Repairing the C0310 will probably clear the C0374.
As for a manual, don't fool around with aftermarket books, go to the GM website and order the Shop Manual for your vehicle.
http://www.helminc.com/helm/product2...149445U72X9ME3