Not the person who tried sea foam, but here is what I know. I had a 99 Montana that started shifting hard and my mom's 2000 Lumina had the same problem.
In both cases it required replacing what my trans guy called the force motor which (i believe) is like the trans fluid pressure regulator. Also they had to replace the shift solenoids. I think he told me if a solenoid hangs up then the regulator jacks up the pressure to compensate and it causes it to shift real hard. In the case of my Montana it also took out the torque converter.
If you don't wait too long it can be fixed without a complete overhaul.
I would have a trans shop look at it. The guy I deal with just plugged in his scanner and he knew what was wrong.
hope this helps. tom