My garage door shake and sits crooked

I have a couple of garage doors that are about 20 years old in the house I just purchased. The track do not look like they are loose or anything and the door openers feel like they are on solid as well. However when the doors close they visibly rock side to side on their way down and are noisy. One side or the other ends up having a noticable gap on the bottom when closed.
My first guess is that they are not balanced correctly because the springs look pretty new so they must have been replaced at some point recently. They attach at one end of the cable with a bracket with a bunch of notches in it to adjust the tension, but I have not had any success playing around with the tension because I am just guessing.
Is there a procedure for adjusting the tension on the springs so the doors don't rattle on their way down and so they end up flat when closed?
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