I'd bet it's your tires. I've had this problem with my 'tour and my wife's Montana (maybe it ain't really a minivan
)
After much biting, scratching and general getting the run around I finally found a shop that admitted I had tires out-of-round on both vehicles. Since they were replaced things have been nice and smooth.
Good luck with it. From my experience, most shops won't admit to tires being out-of-round. I guess they don't always get their money back from the manufacturer or something like that.
