I have been through this.
On my car it was the transfer valve that was bad.
To find out if the AWD is working, the car needs to be on a lift or jacked up and on stands with all four tires off the ground.
Then the car is started and the trans put into drive. All four wheels should turn at the same speed. A little brake pressure should not stop the wheels and a little hand brake pressure should not stop the rear wheels.
Your RWD is not working when you have torque steer under heavy accelleration. Not a real fun experience eh? You could end up off the road.
I have no experience with getting the dealer to replace the whole transmssion. Maybe you can get him to start the clock over again on the extended warantee for the trans.