I have a 2000 GMC Jimmy. My service 4WD started coming on about a month ago. I took it to a mechanic who replaced the front differential valve on the front transfer case and also replaced the vacuum switch for the 4WD. Now every other time I start the vehicle, sometimes the service 4WD light comes on and sometimes is doesn't. When the light doesn't come on, I have 4WD. That is I can engage 4WD high and low and also Auto 4WD. I think it is something electrical but I have already spent $300 in labor and parts. Does anyone have any suggestions about what could be causing it?