Rep Power: 0

I have a 1996 LX automatic transmission Civic (all stock and around 144k miles) and it has a very harsh down shift going from 2 to 1. It usually happens when I let go of the gas and am rolling to a stop. Around 20mph it starts shaking, it shakes the entire car. I can make it stop by giving the gas a little tap, or slowing down quickly. As an experiment (because I have so many discussions on this topic no one seems to be able to come to a common answer) I drove around my neighborhood in D3 instead of D4, and I noticed it did it slim to none. So is this a transmission problem, or maybe a solenoid problem?

-Thanks
Anthony

P.S: The check engine light is on, but when i had the code checked (I can't remember what it was) the guy told me it's most likely the O2 sensor. I know it was doing this before the light came on, I just can't remember if it got worse or not when it did.