There is a nut for a seat bolt under the car that has an access to it thru a hole in the frame, the rear outer bolt( this applies to both seats). It has a washer attached to it so it wont fall thru a box end wrench. I would take it out and find out why it rocks regardless. Maybe it is missing or such and could be dangerous in a wreck.
They do sell a bracket to fix a sagging seat, but I am not sure of a rocking motion. Seems like maybe a bolt is missing and possibly there is something wedged under it now making it rock.
My advise is to pull it out and see if the issue is easy to spot. And it also gives you a chance to clean under it and find stuff that has been under there for years!
They remove rather easily with 4 bolts, be sure to lean it back and disconnect the sensor plug from it before you pull it out. This tells the car if there is a person in the seat, thus the seat belt light will go on if the belt is not buckled.
Let me know what you find dude.....Chas:cool