Tire Bulge problems of the 1998 Ford E-350

We have experienced four catastrophic tire failures in less that one year. All were on 15-passenger Ford vans used to transport students. Each failure involved the rear tires. There was no warning. The tire exploded so violently that the van shook. The tread came completely off and destroyed the rear heating. We just noticed that a fifth tire had a large bulge indicating that a failure was about to happen. About 8 years ago one of our vans had a blowout and rolled killing one chearleader and seriously injuring several more. I do not know if that tire was a goodyear workhorse. What can your agency tell me about the safety of this brand of tire? are there any load e tires that you have not received complaints about?( dot number: ).