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check for 4wd actuator problem Check for power activation and check for vacuum line connection. Lastly check that the hubs are correctly selected as if one side has not properly engaged it will feel like there is no 4WD
check the vaccum hses on the back side of the tire if there ok then with the engine running the selector in 4x4 pull the hose off if you hear or feel vaccum on the hose the your iwe is most likely bad on a 07 they cost about 25.00
Check transfer case wiring, possible loose wiring/open circuit. If your other lights in dashboard work such brake light, seat belt lights, engine light than it is not a fuse. raise all for tires from the ground and engage the four wheel drive remember to have in neutral and put in gear either 4L or 4H and see if the wheels in front move.
First thing to make sure of is to check for a blown fuse. The fuse location is marked 4WD and it should be a 25 amp fuse. If the fuse is bad, the lights on the 4wd console wont light up. If the lights do light up, then you can move to the transfer case. On top of the transfer case, toward the rear of the truck is a switch that has a 2 wire connector. That switch sends power to the front axle actuator when you try to enagage 4wd inside the truck. That could be bad. The switch is threaded into the transfer case and is still available through GM. If the switch is ok and the wiring that attaches to it is ok, then you can move up to the axle. Threaded into the front axle is the front axle actuator. These actuators are prone to failure in older trucks and is very prone to cold weather failure. You can test the actuator by removing it and then WITHOUT the truck running, put the truck into 4wd and then turn the key to the hot position. The plunger inside the actuator will extended outwards. More than likely it will be one of those 3 things. If it still wont engage, you may have a short in one of the wiring harnesses associated with the 4wd system. Also, you may have a damaged transfer case. To check the transfer case, put the truck in 4wd without it running and then crawl underneath and see if you can turn the front shaft by hand. If not....it's good. If you can....then the shifter forks inside the case are worn or damaged. I hope this helps.....start small and work your way up!