'03 Galant: Why rough idle at stops, but only in "Drive"
 

My '03 Galant has recently developed a moderately rough idle and vibration when I come to stops, but it goes away as soon as I shift the automatic transmission into neutral or park. Then it's very smooth & quiet. The rough idle is only while in "drive" at stops. I just changed the plugs today, the engine would seem to be tuned right anyway since it's smooth & quiet in neutral or park.
What might some of the reasons for this be?

trans oil.
Clutch chatter.
Torque sensor.

Thanks.. but now I'm wondering: I neglected to mention, besides changing the spark plugs I did check the trans fluid, none registered on the stick at all. It ended up needing almost two quarts poured in to reach the normal range on the dip stick. I was thinking, then, that was the prime suspect. But I've driven the car around town since adding the fluid, and still get the shaking at stops. Since I probably drove the car a month or so with the low fluid, although not a lot of miles, the first thing I'm wondering now is if it damaged the transmission in any way, to where the shaking is still due to the transmission. It otherwise shifts and sounds fine, and the person at the auto parts store told me transmissions are more forgiving with low oil than engines.
Can I safely assume the shaking is more likely to be caused by one of those other things, then?

frankly, you don't deserve a vehicle.
take it to a professional to, tell them what you did and pay them a lot of money to fix it.

Yeah right, wise guy. You weren't even able to answer my question.
There's a good reason I wasn't able to check the fluid lately but I won't go into that. There's also apparently been a slow leak, but the car's been parked over a dirt area lately so I hadn't noticed.
The car seemed better today and the shaking was less, so maybe I got lucky.
It shifts and drives great at the moment.
Stuff happens. It would be great to have all the time in the world to devote to perfect preventative maintenance on my vehicles, but some people's lives get real busy and distracted with other things, in case you couldn't imagine.