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I am a mechanic but not a tire guy but I have to chase shake and vibration issues on a weekly basis so bear with me. Shake and vibration are pretty closly related so a little more info may be needed. When it shakes does it change AT all when letting off on the gas pedal instantly or does it stay the same until you reduce it to a lower speed gradually? If it changes when the load is taken off instantly you are chasing a drivetrain problem and not the tires or wheels. Choices here are tight or loose u-joint (one sticking in a plane may not be evident unless you pull the driveshaft and feel it), loose pinion in the differential , or something like a bad rear transmission mount. if your problem is the latter and you have to get rid of the shake by reducing to a lower speed and it gradually goes away you are back to wheel, tire OR unbalanced driveshaft. If you jack the car up you want to spin the tires and look for side to side runout and especially up and down runout . if you have it on a hoist (preferable) run it in drive and watch the rear wheels. The catch here is that a tire can be balanced with runout and tire shops miss this. I have seen customers have tries balanced that where badly cupped and the tire guys never said anything to them. Also do you feel it in the steering wheel? If yes or know it gives you a direction if you are chasing backs or fronts. The important point here is little to no up and down runout. The tire needs to be round. Sighting the tread when spinning will tell you side to side runout. Sorry for the long post but I feel for you. These can be hard to chase.
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