One problem related to tire bubble has been reported for the 1998 Toyota Sienna. The most recently reported issues are listed below.
I replaced the firestone tires which our Sienna came equipped with when new because the vehicle began steering very poorly, to the point of my becoming fearful of driving on them. I rotated the tires myself (fronts to rears) and found that the steering problem was solved. Of course I then knew that at least one of the rear tires was no good. When I purchased the four new tires, I asked to examine the old firestones just removed. Two employees of the discount tire and myself found bubbles indicating that separation was occurring. I kept the tires. They may be examined at my home. ( dot number: tire size: p205/70r15 ).