[Z15]

You are going to have to have a GM dealer trouble shoot this. It could be one of 3 possible things. The switch is bad (if your lucky and many do go bad, more delphi junk), the control brain box under the dash or the acutator in the transfer is bad. The switch is cheap but the other 2 items are pricy (as much as $400 for the actuator) because they are electronic motors, so you don't want to change the wrong thing. There is a methodical way the dealer can solve this without buying unecessary parts.

Bet the reason it was sold it that this problem surfaced.

I had similar problem with my 2001 GMC when it was new, it turned out to be the under dash module. Later the swtich went bad.

I would be very hesitant to take it anywhere but a GM dealer. Some of the other shops may not have the knowledge to trouble shoot this problem and it could end up just prolonging it. Feel them out before you give the the truck to screw with.