If your vehicle is diesel with an allison then it's a neutral saftey switch if it's regular 5.3/6.0 the you could have a faulty vss ( vehicle speed sensor located on the transfer case and the speedometer might be s seperate issue they been known to short and you just have to replaced them .
I would replace your vehicle speed sensor to start out with.
SOURCE: 4X4 inop on 2003 GMC Sierra 2500HD
I have found that when you are going to engauge 4x4 high, it is best to only be going about 3 miles an hour. Not faster, not stopped. It likes some movement to help it mesh. I've had perfect shifts since I started this. Hope it helps. Wade.
SOURCE: 2003 gmc sierra duramax 2500
Problem with Diesel Fuel System. Gmc 2002 duramax 2500 HD 4x4 will start
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