If your vehicle is showing grinding noise when trying to engage 4-wheel drive, here are the most likely causes and the parts you'll need to fix each one.
Many modern 4x4 vehicles use vacuum-operated front wheel hubs. A vacuum leak or failing actuator causes the hub gear to only partially engage, grinding against the axle splines while driving.
Attempting to shift into 4WD Low while the vehicle is moving, or shifting into 4WD High at excessive speeds, will cause the gears to grind. The vehicle usually needs to be stopped and in Neutral to engage 4WD Low.
The electric motor that shifts the transfer case can become weak or corroded over time. This leads to slow or incomplete gear engagement, causing the gears to grind during the shift.
Old, contaminated, or low fluid in the transfer case prevents smooth gear synchronization. This lack of lubrication leads to harsh, grinding shifts when engaging the front driveshaft.
Worn front CV joints may be silent in 2WD but will produce a grinding, popping, or clicking noise under load once power is actively routed to the front wheels in 4WD.
Over time, the internal shift forks, synchronizers, or drive chain inside the transfer case can wear out or stretch, causing severe gear grinding and slipping when 4WD is engaged.