It seems to sound like your torque convertor clutch (lock up) is not disengaging as you slow down, and the transmission is dragging the engine down as you slow down. at highway speed you should notice a 200 rpm reduction ( one unit) on your RPM guage, a second or two after shifting into top gear. The RPM should jump up (increase) one notch If you increase a bit of throttle pedal. By repeating this procedure, you should see, (and maybe feel) the engine speed changing, this will let you know if the convertor lock up clutch is operating properly at cruise speed. Try slipping the transmission into neutral before the shudder occurs, (as you let the car coast to a stop,) and see if the problem disappears. It's usually activated by engine vacuum in older cars ,but is probably electronically controlled by the trans . computor in more modern cars. At least you can save $ by giving the dealer the exact problem to fix! By the way have you checked for transmission error codes? I would suspect a P1529 error should be there, and just clearing it would probably end the annoyance. (code readers usually sell for $ 30-50 on sale at auto part stores.) far less than the shop charges to check it, and It's an easy 2 second plug in to check it yourself....
