It sounds like your tranny sh!t the bed.
http://www.cadillacforums.com/forums/cadillac-sts-forum-2005-through-2012/652234-rough-ride-low-rmp-range-help.html
How did he drop the pan and determine it was 2 qrts low. There is no fill stick to determine it. You should NEVER go by a set level. You run it and check by how much fluid it takes before it pours out of the fill. Also a flush wouldn't magically make it work. Your trans is either failed or not.Chinny said:Earlier in the thread I said I had a mechanic check the fluid, drop the pan, found out it was low 2 qts and had metal in it, and change the fluid and filter. I did just pick it up and it is riding rough, I am sure the tranny will fail again sooner or later.