If you put the ATX selector in 2nd and you can come to a stop without the engine stalling then the TC clutch isn't stuck 'on'. (Agreeing with your CEL code) At a minimum, you might want to replace (drain/flush) the ATX fluid to see if that helps. Try driving the car (easily) on an open road to see if you can feel the TC lock up. You should see/feel the tac drop about 500 RPM shortly after it goes into overdrive/4th. From my experience, TC lockup doesn't happen with OD off. Speaking of OD, is you OD light flashing?