Google "Chevy Shake", I have a 2018 Chevrolet Silverado LTZ 4x4 and it happened to me. There is a bulletin out on it for 2014-2018 models. They will start with flushing out the transmission and if that doesn't work then they will work thru to the converter/transmission. They say it could be the fluid used in it. They flushed mine and I've put about 5k on it and haven't had any issues since. If i remember when i get to truck I'll send you the bulletin number.