My truck will not go into four-wheel drive. No SES or other lights are on the dash.
There is usually a certain procedure that is necessary for the switch to four wheel drive, such as selecting the neutral option on the transmission, or being stopped. If you have tried these and the vehicle still will not switch, there could be some electrical fault blocking the action. There may be a code stored, or lack of data from a sensor, that is disabling the four-wheel drive. Have a trained mechanic, such as one here at YourMechanic, come to you and diagnose any potential electrical issues.