Our Chevy s10 4 wheel drive will not engage in 4 hi or 4 lo. The push buttons for 4 hi are constantly lit, even though it"s not actually in 4hi. when you push the button for 4 lo or 2, they flash and then return to 4hi being lit. you cab hear a noise when you try to engage it in 4hi or 4lo, but nothing happens.