To the original poster dennislanier,
There was a recall on Michelin tires made during a certain time of which were prone to blowouts like you describe with your sidewall bubble/tumor. Get the production number off your tires and call a Michelin dealer. You might get your tires replaced for free under the recall, both the tires & labor charges covered. You can also just drive your rig to a Michelin dealer and have them find the numbers for you. Even if your tires are not covered under the recall, they may pro-rate your bad tire.