ORIGINAL 4X4 FRONT AXLE ACTUATOR REPLACED BY AUTHORIZED G.M. DEALER. PART WAS INCORRECTLY INSTALLED CAUSING FRONT AXLES (HUBS) TO REMAIN LOCKED DESPITE BEING IN 2 WHEEL HIGH. DOING 70 MPG ON HIGHWAY, REAR U-JOINT GOT TOO HOT AND SEIZED CAUSING TORQUE TO BREAK TRANSFER CASE IN HALF AND ALLOWING DRIVESHAFT TO TEAR LOOSE FUEL LINES AND TEAR HOLE IN FLOORBOARD DIRECTLY BELOW DRIVER SEAT. FAULTY ENGINEERING OF NEW ACTUATOR SWITCH DESIGN DOES NOT PROVIDE WARNING WHEN HUBS ARE LOCKED. EXPLOSION OF TRANSFER CASE CAUSED NEAR ACCIDENT AND MINOR FIRE.