OUR 2001 BLAZER 4WD FAILED TO ENGAGE . WE WERE TRAVELING EAST ON HIGHWAY 36. IT WAS SNOWING. I PRESSED THE 4WD HI BUTTON AND IT CONTINUED TO BLINK. WE WERE ON A NARROW TWO LANE HIGHWAY AND HAD NO WHERE TO PULL OVER. THE 4 WHEEL DRIVE FAILED TO ENGAGE AND THE BLAZER, WITHIN SECONDS SLIPED OVER THE CLIFF. THE FRONT WHEELS DID NOT LOCK IN. WE ROLLED DOWN THE CLIFF APPROXIMATELY 50 FEET AND HIT A TREE.