I had this very same problem with my B2200, I keep taking it to the tire shop every weekend cause of a shake at hwy speeds, they balance it everytime, checked to see if it was out of round and rim true. But it would still do it. Finially I stoped taking it to the mazda dealer recommended tire shop(seeing I just bought the truck), and went to a Tire shop that I trusted and had been around for 20 odd years, he ask if anyone has ever dismounted the tire, No was my answer. They took the tire off and it was full of that roadside tire repair foam, he was saying that after they repair a tire that foam is to be washed out, or it turns to a liquid and throws the wheel off balance, and a balance machine won't notice it. They washed it out, dried it, put it back on, balanced and never had the problem again.