I was pulling a 7500lb trailer with my 2020 GMC 3500 Crew Duramax 4WD with 78k miles at 55mph on a California freeway and the rear wheels locked up for approx 30ft then released then locked up again for approx 30ft then released. The trailer fishtailed both directions violently and I was able to get it pulled over to the shoulder. Put the tranny in park. Then I put in drive and reverse and clunking/grinding from 10 speed Allison tranny. It would move in either direction but clunking/grinding. Towed to dealer. All dealer has said so far is "internal transmission failure". In hindsight a few miles back when the tranny would upshift to a lower RPM below 2000rpm it would hard shift. This happened at approx 50-55mph. It did that a few times before it locked up. Very scary with a trailer. I almost had a bad wreck in heavy traffic. I'll keep you posted on what was found etc.
