@Slimtwitch i am having the exact same thing with me also, i have a 2017 silverado z71 crewcab 4wd and on a 4/6 aswell. ever since i got it dropped it started making the grind noise 70mph and above on deceleration. ive been trying for months to figure it out and i just cant. did you ever find out what it was, id really appreciate it!