This may be no help atall, but i had a GMC pickup once, and there is a vacuum switch that was stuck and that was causing my problem. it was around a $300 dollar fix, and that was 8 years ago, so i imagine it is quite a bit more expensive now.
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one real common problem is the 4x4 selector motor on the transfer case wears out. when a position is selected, the transfer case selector motor will over run the position, meaning it doesnt stop in the correct position. you can unbolt the motor and manually select the 2x4 position, then align the motor and reinstall. replacment is required for it to work properly
I dont think you have a transmission problem , i believe you have a transfer case problem. These are repairable but sometimes is more cost effective to replace. I would offer my services but am way to far away for that to make sense. Let me know if there is anyway i can help.
Your transmission should be in neutral when shift from 4x4 low to high. Also the brake needs to be depressed. If you have done that I would say it is a solenoid in the transfer case or the relay that drives the transfer case solenoid.
make sure your 4WD switch is working.and fuse is okay.because your 4WD is control by 4 electric shift motors in which drive a rotary cam which moves the mode fork and range fork inside the transfer case to select between 2WD HIGH A4WD - 4WD HIGH AND 4WD LOW RANGE.a shift motor sensing plate on the shift motor monitors the range in which the transfer case is operating a transmission range sensor monitors the gear position of the transmission.which must be in neutral to shift into low.a pair of solenoids one for 2WD one for 4WD.mounted on the upper right firewall route vacuum to the vacuum motor which engages and disengages the center axle disconnect collar in the front axle assembly.a 4 x 4 shift motor relay module contains two GEM controlled relays which shift the transfer case shift motor between 2WD and 4WD modes.a solid state clutch relay activates the A4WDclutch inside the transfer case.