I thought I'd create a thread with a possible fix for your high speed vibration. I've run into this issue with two seperate cars. I had a vibration that would start around 70mph and get worse the faster I went. It would be real bad one day and not so bad the next. Hitting the brakes would make it worse. I was told by multiple mechanics that the brakes would not cause that kind of vibration when I was just cruising on the highway. I checked all the suspension/driveline components. Had the tires balanced. Put a totally different set of wheels and tires on. I had no idea where else to look. What I eventually found was a stuck brake caliper piston on the front left brake caliper. There are two pistons on the front calipers so they will still function somewhat normally. The problem it the second piston won't back off so the pad stays really tight to the rotor causing lots of heat and vibration. It warps the rotor from the heat too so the rotors are usually cooked. It's definitely something to check. Did the front brakes and changed the caliper. No more vibration! Smooth like butter! Definitely something to check if you have high speed vibration.
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I would assume u should be able to smell this issue right away.