I had the same problem on a 90 gmc 4x4. I found that the wires for the auto locking hubs run close enough to the exhaust on the passenger side that they melted and shorted out the 4wd fuse ( yes there is a fuse marked 4wd in the fuse box.) If this fuse is blown and when you replase it blows again this could be the problem. Good luck
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My 2005 Sierra 4x4 did the same thing while running down the interstate on vacation. Scared me to death, thought it was going to shift into 4 wheel drive @ 80 MPR. Had to take to shop and they found a censor bad and they replaced it. Don't know cost, because truck was still under warranty.
Typically this means that the truck is trying to engage 4wd, and can't. moving slightly should allow it to. If it doesn't, you have a problem.
Some models will flash the 4x4 light when you are in 4wd low. I am assuming it is a new thing, so this probably doesn't apply.
Turn your wheel fully to one side, and if it doesn't "grab" as you move, your 4wd is not functioning, and you need to take it to a mechanic, or grab a service manual, depending on your skill.
Hello David. The power for the switch is a 5 volt supply from the powertrain control module. To diagnose this efficiently you will require a scanner capable of reading the datastream and codes in the system. Very difficult any other way sorry to say. Hope it goes well for you.