4 wheel drive service light

[Hog Breath]

Just recently bought a 99 Yukon, right off the bat, the service 4 wheel drive light has came on, checked fluid levels, they are all right, any ideas on what to do next?

 

[SunlitComet]

need to get a high end scanner to plug in and read the "B" codes. 1998 was the start of diagnosticable 4x4x systems.