ok, you need to check the 4wd actuator that goes into the front differential. make sure you are getting power and ground at the plug-in when it should be in 4wd. If you do then the actuator is probably bad. the actuator acts as a heater on a fluid inside the actuator. the heat expands the liquid into a gas and pushes the end of the actuator into the 4wd gear that locks the axle in. When the axle goes in and locks the 4wd light comes on. It is very common for the actuator to leak the fluid out after time. you can replace it by unplugging id and just unscrew it then just screw in the new one and plug it back in. Like I said though check it first. don't go out and buy one and then find out it doesn't work because there is something else that's causing the problem.