A drain and refill is not enough, that is only 4 qts if you're lucky, so 4 out of 13 qts.You're right the "shudder" is only when shifting into the last gear.
It's been more than two weeks since the drain and refill and I still have it.
You can disconnect the return line from the radiator, run it into a pan, and run the car until it starts to sputter. Helps to have someone in the car to shut it off while you're watching so it doesn't pump air for more than a second. Then drain what's left in the pan, and drain the torque converter. That should get you around 11 1/2 qts. Be sure to measure it, old 5 qt oil jugs are good for that.
$80 for new fluid cost the same as one hr at a shop, and most shops consider a fluid change to be just dropping the pan, which you have already done.