Take your car back to the dealer to have it checked. Its possible there is a computer update or a known issue that a gm dealer would have access to. Do not change or add anything to the transmission. If there is an internal transmission failure and the fluid has been changed or anything added to it, gm will request a fluid sample from the tranny and deny the warranty claim.
〉 Answered on Sep 9th, 2013 by Helaine Kurot, Owner/Technician at 360 Automotive
David, Your 2012 GMC Acadia is most likely under warranty if under 100,000 miles. Check with a local dealer. Shifting hard could be caused by a number of things including high line pressure, a bad sensor or an internal transmission problem. Getting the vehicle diagnosed with a scanner and pressure gauge is where to start.
〉 Answered on Sep 9th, 2013 by Maryann Croce, Owner at Croce’s Transmission Specialists