Tire Bubble problems of the 1998 Toyota 4Runner

4 dunlop radial rovers 235/75 r15 installed 2 or 3 years and 30,000 miles ago on my 1998 Toyota 4runner. I always maintained recommended pressures and just rotated them myself last week after our previous las vegas trip. On our way back from las vegas July 19, 2004 2 pm near fontana the car began to shake progressively until it was undriveable above 45 mph so I pulled over to a small turnout. I changed the left rear tire that I found had a major bulge (3 inch diameter bubble on sidewall which continued another 3 inches into the tread. ) changed it to the spare and car still shook when driven. Found out the front right tire was worse, about 1/3 the circumference was bubbled making the tire look egg shaped from the side. Looks like more than tread separation, possibly broken belts since the bubbles had air pressure. Drove 10 miles to a discount tire center and had 2 tires changed at a prorated charge (kumho venture) they kept the old tires. 45 miles later and just before my exit on the freeway the car began to shake. Luckily we hit traffic near our exit and we were going 10 mph when the third dunlop went totally flat. I will have that tire and the 4th one changed today.