1968 Chevrolet C Series

THE REAR TIRE ON MY PICKUP FAILED WHILE TRAVELING ON THE FREEWAY AT APX 70 MPH. THE LEFT REAR TREAD SEPARATED FROM THE CARCASS AT SPEED. THE REMAINING 3 TIRES HAVE ON AVERAGE 5-6 MM OF TREAD REMAINING. THE TIRE PRESSURE WAS CHECKED THE DAY BEFORE 53 PSI AND I INCREASED THE PRESSUE TO 60 TO MATCH THE OTHER TIRES. THE VEHICLE WAS PARTIALLY LOADED WITH APX 600 LBS IN THE REAR. THE TIRES WERE ORIGINAL EQUIPPMENT "TAKE OFF" TIRES AND RIMS FOR A 93 F250. THE TIRES AND WHEELS WERE PURCHASED AS A MATCHED SET WITH LESS THAN 600 MILES WEAR, FROM A FORD DEALERSHIP IN LATE 93 OR EARLY 94. I AM NATURALLY CONCERNED FOR MY SAFETY WITH THE REMAINING 3 FIRESTONE TIRES. THE TIRE DEMONSTRATED NO BULGING, EDGE SEPARATION, "THUMPING" OR FLAT SPOTS PRIOR TO FAILURE. THE TIRE DID SHOW SIGNS OF A SLOW LEAK WITH A LOSS OF 2-4 PSI A MONTH WHEN SET AT 60 PSI. REPEATED INSPECTIONS FOR NAILS, SCREWS, PUNCTURES OR SIDEWALL DAMAGE WERE NEGATIVE. I ATTRIBUTED THE TIRE PRESSURE LOSS TO A BAD VALVE STEM. THE WEAR PATTERN ON THE REMAINING TIRES IS CONSISTENT WITH NORMAL OPERATION. I USE THE VEHICLE RECREATIONALY AND NOT FOR WORK. PLEASE HELP, WHAT SHOULD I DO??? (DOT NUMBER: VNOR1WT103 TIRESIZE: LT23585R16)