Tire Blowout problems of the 1996 Ford F-150

Bought a set of tires. At 17,160 miles had a blow. Took back to dealer had to buy a new tire. At 19,000 miles had another tire separated. Had to buy anew tire. At 20,000 miles had tire blow out. Had to buy a new tire. At 26,000 had another blow out at 65mph. Did not wreck. But went to ditch . Dealer refuses to do any thing about this. What can I do.

While driving about 70 mph started feeling vibration. Then, left rear tire had a blowout wich opened the tire rim to rim . Ford, f150, 1996, p-235/75r15, dot# anhllv1138. Tire was sent to goodrich who stated that the tire had been under flated which along with the heat caused it to rupture, consumer states he strongly objects to their findings, bf goodrich tire.