NHTSA Complaint #11557188 — 2021 GMC SIERRA 1500

5 days after purchasing and While driving down the interstate at 80 mph, there was a loud clunk at the same time the back wheels locked up for a split second, long enough to burn the tires and throw everyone in the vehicle forward then the vehicle continued on. It happened three more times, we trailered it and brought it to the dealer. They said nothing is wrong with it. However, I was able to locate the previous owners who stated they'd been bringing it to several dealerships to have it taken care of for the past six months and every dealer said that nothing is wrong with it. But the same thing happened to them for six months. One mechanic in Idaho even replaced the motor trying to fix it and it still continued. The dealer I purchased from never mentioned it having a new motor, and I pulled the Carfax report and it shows several times it was in the shop including the motor replacement, but the shop visits aren't detailed it just says maintenance. Several mechanics we've spoken with say it's in the transmission, drivetrain. But the dealer refuses to do anything unless it'll replicate when they test drive it. But they don't test drive it on the highway or for a long enough time. Multiple owners now in multiple states the same problem and no one will help. This could be a catastrophic incident if the roads had been a little wet or someone was close enough behind us when it happened it would have been a serious car accident, if not worse. I am not sure you can help but Im reaching out to everything that I can find.