getting trouble code flashing and going off

Light on dash blinking when driven 15 miles in 2-wheel drive but doesn't come on when in 4-wheel drive. What does it mean? My Mitsubishi Montero limited 2002 has a little diagram of the wheels that light up either the bottom two for 2-wheel drive or top and bottom for 4-wheel drive. In the middle is another light that says C/D on one side and Lock on the other this light starts to blink if I drive in 2-wheel for 15 miles but will turn off and not come back on if I turn the car off and put it in 4-wheel drive. Once I drive it in 2-wheel for 15 miles it will start blinking again. What does this mean and how do I fix it.