Same here, truck always had a shake around 75ish. Got a little more pronounced after I did a rear flip kit, changed to shorter rear shocks & put on 22's.. Finally after about a year of dealing with it, I decided to take it to my buddy who is a GM tech & has been using the hunter machines since they came out. He road force balanced my cheap Ironman 305/40/r22 tires & I tightened the rear shock bolts up again for good measure. Shake is gone & I dont hate driving my truck anymore.. Buddy said these trucks & the C7 vettes, depending on what suspension setting its on, are very finicky with rear shocks & needing roadforce balancing.