hello i have a question about an 03 gmc yukon VIN. V with 73000 miles // the 4x4 switch shows neutral upon start up sometimes // it isnt in neutral and can be driven but 4wd cannot be engaged // sometimes it is in 4wd low on start up and is actually in low // can you tell me how to diagnose this / I have a wiring diapram and have checked some of the grounds and the 5 volt reference but dont know where to go from here ; thanks