A faulty front axle actuator will prohibit your truck from going into four wheel drive. One of the symptoms would be that the four wheel drive switch just flashes when trying to engage. There also may be codes stored in the transfer case control module.
To test yours, simply disconnect the wiring to it and remove it from the front axle by unscrewing it. Reconnect the wiring and engage the four wheel drive, the actuator should extend. If it does not and the wiring is ok, the likely fault is the actuator.