I have a 1996 Chev Z71 with push button 4wd. The truck will not fully engage 4hi or 4lo. When i push the button the light on the button flashes and a clicking is heard from under the dash. Every so often you will hear the 4wd lock in and the light will stop flashing. When this happens the 4wd still does not work only the rear tires will move. Someone please help me out i have no ideas.