I've searched and can't find my issue. 2004 base avalanche 1500 (new to me), occasionally the service wd warning will come on. When it does my 4wd (or auto 4wd) light will not light. eventually I can get it to engage but sometimes on auto it will engage, work fine, then the light will go off (not sure if it reverts to 2wd or not). I know it's never a good idea to throw parts at something without totally knowing the problem BUT my scanners don't do trans codes (and I live in a rather rural area). I am planning on checking and cleaning the xfer case grounds (and maybe adding one) . I also plan on replacing the encoder motor ring. What else would be a likely problem area I figure grounds and ring are a common fault and worst case I'm out a little time and money BUT have a better baseline of the 4wd service done on the truck.