1990 Silverado 3500 7.4L - we have recently had over 20 inches of snow and my driveway is solid ice but the main roads are in good shape at the moment. Calling for 2 to 4 inches of sleet tonight. So this morning I put the truck in 4 wheel drive to back out of my driveway and when I stopped the light on the floor shifter went back off, thought maybe it was just a bulb, put it out of 4 wheel drive and went to work. When I got home this evening I found that the 4 wheel drive is not engaging at all. Several years ago I had the (transfer switch?) replaced. The mechanic had bought the newer design which was supposed to engage faster but found that it would not fit on a 3500 and had to replace it with an original OEM part. Recently it has been taking longer for the 4 wheel drive to disengage or at least it has been taking a few miles for the light to go off. My first thought is a vacuum line somewhere. Does anyone have a diagram for the vacuum lines? Looking for any suggestions on where to start to diagnose the problem and hoping for a quick fix. Thanks.