[Ironangel]

Check the backlash, and check for end play on the pinion shaft. The GM 12T should have .006" to .010" of backlash and zero end play...I'm thinking you have a rear end that was never correctly set up, and/or a pinion shaft that has end play due to overtightening the initial preload on the crush sleeve. All repairable if you pinpoint and address the problem now. Jack the rear axle up until both wheels are clear and set it on stands. Pop the cover and capture the lube in a clean pan for inspection and reuse if clean. Pull both axles just far enough to clear the splines in the carrier and attach a mag base dial indicator to check the backlash on the ring gear. You can get under the truck and push in and pull out on the pinion yoke using the drive shaft. There should be zero end play. If there is end play plan on buying a $4 crush sleeve and a $3 pinion seal and re-set the pinion preload. I took the whine out of one of my 72's by simply tightening up the loose backlash by swapping shims from the left side to the right...Here's a manual, https://www.randysworldwide.com/wp-c...l_Complete.pdf