The cheapest thing to try right now is to get the wheels rebalanced and also see if the tires are wearing odd (aka cupping, scalloping, bald inner or outer sidewalls).
Do you have aftermarket wheels? If so what size? The larger your wheels the more chance you have of bending them if you have hit a huge pothole or road hazard. Run your hand around the lip of the wheel (both inside and out) and see if you feel any indentation. I have had the smallest bend on my older wheels that I barely felt but my sponsor was able to find it.
The other thing that it could be is bad shocks, suspension bushings or just worn out suspension components which can wear your tires weird.
Usually anything that shimmies and shakes at a certain speed is a bent wheel, bad tire or out of balance wheels/tire combo.
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