If your vehicle is showing vehicle refuses to shift into 4wd or 4-low, 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 often fails due to corrosion, moisture intrusion, or lack of regular use.
Vehicles require the transmission to be in Neutral (and often stopped) to shift into 4-Low. If this switch fails, the computer doesn't know the vehicle is in Neutral and will block the shift.
Old, contaminated, or low fluid can cause the internal mechanical components of the transfer case to bind up, preventing the shift forks from moving into the 4WD position.
Modern 4WD systems rely on accurate wheel speed data to safely engage the transfer case. A failing ABS sensor can cause the system to disable 4WD to prevent drivetrain damage.
Severe wear or breakage of internal components like the shift forks, chain, or planetary gears will physically prevent the transfer case from engaging.