If your vehicle is showing vehicle is stuck in 4wd and won't shift back to 2wd, here are the most likely causes and the parts you'll need to fix each one.
The electric motor mounted to the transfer case that physically changes the gears is highly prone to failing, seizing from corrosion, or getting stuck due to infrequent use.
Driving in 4WD on dry pavement or installing mismatched tire sizes causes the drivetrain to bind up with tension, physically preventing the transfer case gears from sliding out of 4WD.
The electronic module or the dashboard switch that commands the 4WD system has failed, meaning the electrical signal to disengage the transfer case is never sent.
Even if the transfer case successfully shifts to 2WD, a vacuum leak or mechanical failure at the wheels can leave the front hubs locked, causing the steering to bind as if it were still in 4WD.
A bent shift fork, broken synchronizer, or severe internal mechanical damage is physically preventing the transfer case from shifting out of 4WD.