97 Grand Charokee - Wheel Diagram
 

My drive wheel diagram(next to the cigaratte lighter) has always shown 4 wheels lit up until today and now only has the back wheels lit up. I switch the quadra trak shifter from all 4(normal) to low 4 and back to normal again and now-none of the wheels are lit up on the diagram. What doe this diagram really tell you and where does it get it's sensor information from??
Any info would be appreciated.
Thanks in advance
Paul