Your shimmy sounds very much like your wheels and tires need to be "weight matched". When you purchased the tires, it is likely that you had an inexperienced installer mount and balance them (or that didn't care). Look at them, some tires and wheels are marked with dimples, dots, or arrows. They'll usually be aligned with wheel dimple or valve stem (180* of it). I used to work in that business. Higher speeds usually tend to amplify the effects. I used to take the time to weight match each tire and wheel (lightest side of tire with heaviest side of wheel or vise versa), too the point of only having to use a quarter ounce of weight or less, sometimes not even needing wheel weights at all. We actually sent many tires back to the factory/distributor because they were too unbalanced from one side to the other and I mean ALL brands and types. I've done this on tires for little disposable cars with little-bity wheels, low riders, big jacked up trucks with huge wheels and tires, police cars, emergency vehicles, street race cars, stock cars, drag cars, and everything in between. My personal vehicles I always balanced my wheels alone (if real bad I would remove metal from one side) and would do it for others for a small fee. If you're meticulous about it as I am, you may want ask them if they'll do it for you, while agreeing that you'll leave the vehicle there for a specified length of time in order for it to be done right. (oh yeah, tell 'em you like to drive it real fast from time to time lol) There are shops out there with untrained techs that are just out to get your money or don't even know how to do it right, but I really still believe there are shops with technicians, who actually care and take pride in what they do.... you just gotta find them!
Hope this helps you get a smooth ride!
