Tire Tread/belt problems of the 1999 Ford Econoline

The first incident occurred 4/14/01 on the way back to florida from PA. We were on I-77 traveling approx. 60 mph. When a noticeable vibration started. We slowed down and pulled off at the next exit. On inspection the left inside duel had experienced separation of tread from the tire. Tire pressure had been set at manufacturer's recommended pressure. The tire appeared to hold air pressure. Upon arriving home we took pictures of the tire and returned it to our firestone dealer who pro-rated it. Yesterday (6/17/01 we experienced the same problem with the right side inside duel. This time the tire would not hold any air pressure. Our mileage on this tire was 19,126. We find it odd that we have not experienced this kind of problem with other rigs we have owned. We are very concerned about the other 4 tires since we are planning an extended long distance vacation in July. We have not been able to find these tires on any recall list. (dot number: vo1l1xd398 tiresize: lt22575r16).