Take the tires in and get them spin balanced? At that time they can check to make sure they are both round. Out-of-round tires aren't that uncommon, especially trailer tires. You can check leaf spring quality by measuring the distance between tire and fender. If they're both roughly equal then that's not likely to be the culprit.
I'd bet on tire/wheel out of balance or out-of-round.
'04 MariStar 230VRS/MCX