DRIVING AS NORMAL ON FWY HEARD A LOUD NOISE THOUGHT I GOT A FLAT. STOPPED VEHICLE ON SHOULDER, BRAKING WAS DIFFICULT AND WHITE SMOKE WAS AROUND THE CAR. I GOT OUT AND DID NOT SEE ANY FLAT TIRES BUT I SAW SMOKE BETWEEN THE CAB AND BED OF THE TRUCK. I RAN BACK TO THE DRIVER DOOR AND SAW A SMALL FIRE ON THE FLOOR NEXT TO THE 4X4 SHIFTER FOR THE TRANSFER CASE. I IMMEDIATELY GRABBED MY 3 YEAR OLD DAUGHTER FROM HER CAR SEAT AS THE CAB FILLED WITH SMOKE AND RAN APPROXIMATELY 300 FEET AWAY AND TOLD MY DAUGHTER TO STAY. I JUMPED INTO THE BACK OF THE TRUCK GRABBED A LARGE FIRE EXTINGUISHER AND DISCHARGED IT INTO THE HOLE IN THE FLOORBOARD NEXT TO THE TRANSFER CASE IT WENT OUT AND IMMEDIATELY CAME STARTED UP AGAIN. I THEN CALLED 911. SOME PEOPLE STOPPED AND HELPED ME REMOVE A FEW POSSESSIONS FROM THE TRUCK BED. WITHIN 3 MINUTES THE TRUCK FIRE FLASHED AND WE ALL STOOD BACK AND VIDEO TAPED THE VEHICLE BURN TO THE GROUND. THE WITNESSES SAID WHAT APPEARED TO BE SOMETHING METAL CAME OFF OF MY TRUCK STRUCK THEIR CAR AND THE REASON THEY PULLED OVER IS BECAUSE THEY THOUGHT THEY HAD A FLAT BUT WHEN THEY LOOKED BACK THEY SAW FIRE UNDERNEATH MY TRUCK AND RAN TO HELP. THE FD ARRIVED THEY SHOT FOAM ON THE FIRE BUT IT CONTINUED TO GO OUT THEN START BACK IT TOOK THEM APPROXIMATELY 30 MIN TO EXTINGUISH. I AM VERY MECHANICAL AND BELIEVE MECHANICAL FAILURE IN THE EXHAUST CANNINSTERS THAT COLLECT THEN BURN HYDRO CARBON PARTICLES BEFORE THEY ARE RELEASED INTO THE CAT CONVERTER, SENT EXCESSIVE H