Transmission Noise problems of the 2014 Toyota Highlander

Within the past month or so there were a few separate/sporadic occasions when the transmission of my 2014 Highlander with 82,344 gentle miles on it seemed to slip/not engage for a moment when first starting to travel at a low rate of speed. When the transmission did engage during these episodes it was abrupt and noticeable creating an audible noise and jerked the vehicle. This past weekend the same thing happened except that this time the check engine light came on and the messages "trac off" and "check awd system" were displayed. I brought the vehicle immediately to the dealership I bought it from and that has performed all the service and after they inspected it and ran the codes I was told that although it's, and I quote, "extremely rare for a Toyota with such low miles to need it, the transmission is bad and it needs to be replaced. " this frustrating not only because all recommended service has always been performed, and the car has been well taken care of, but I had complained about a vibration and rattling noise when first accelerating from the under carriage while the car was still under warranty and the dealership could not identify where the noise was coming from or what was making this occur when it was obvious it was originating from under the driver/passenger area at the front of the transmission.