Tire Bulge problems of the 1997 Mercury Mountaineer

About 1 week prior to the August 17, 2000 date, I noticed a change in the way the tires felt while driving at slow speeds. I then had my wife drive the vehicle slowly down the street and I walked behind and noticed a bulge in the left rear tire. I then took it to the firestone store in garland, texas and they said it was a road hazard problem. I knew I hadn't hit anything in the road since I had been driving the vehicle since October of 1999 and it had driven fine until this incident. They sold me a discounted tire and charged me $65. 23 to replace it. I had them move the spare (same dot and size) to the left rear and put the new tire on the front. They kept the bad tire and sent me on my way. Then about November, 2000 the same thing happened to the tire that I had them put on the left rear. I then took it to an independent dealer that sells firestone in irving. They agreed that this tire was failing due to tread separation. They didn't have another wilderness at to replace it with so they gave me a competitors smaller tire and made it the spare. They gave me the tire to keep so I could have it replaced with the others when I could get it replace when they had a replacement. (dot number: w2hl tiresize: 23575r15).