hello i have a 2011 ford f250 diesel with a 6.7l. when the engine is running and i go and try to engage the 4-wheel drive system it wont engage. when i turn the truck off then turn the key on the on position with the engine off, i can hear the 4-wheel drive engage and the 4 wheel drive light will come on. as soon as i start the engine, the light goes off but the 4wheel drive is still engaged. i have to turn the truck off to have the 4wheel drive disengage. so basically i have 4wheel drive but i have to engage and disengage with the engine off and the key on. has anyone ever experienced this and if so what can i do to fix this.