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I have a 2000, and I think that it doesn't have TM. Also I had the tuner installed by the place were I bought it from and they maxed out the shift preasure and shift points.
the plastic piston is in the 1-2 accumulator. you can pull the housing down to check it without pulling the valvebody. here are a few pictures for referrence.
Here is a pic of mine, although it never got to the point of causing any bad effects, but it was coming, I'm sure. Notice the 3 cracks around the hole.