99 Suburban-"Service 4WD" Light

[cyoung]

I've been using the 4wd for several days in the snow. Worked fine til yesterday AM when I started it up and the "Service 4WD" light came on. I couldn't get it in 4 hi or low or 4wd auto. 2 wd only. I drove it that way during the day then when I started it up to come home it had put itself into 4wd auto and I could not get it back into 2 wd. Any syuggestions where to start trouble shooting this?
Thanks!

 

[a_kelley]

Check operation of the shift motor, grounds by the transmission, look in service manual and a half decent scan tool will tell you the problem