caster and toe. The alignment itself would not cause shaking, which is more attributable to the status of your wheels & tires (eg roundness and balance). However if the alignment is not corrected it will affect handling and wear on the tires and suspension components, which will eventually lead to further shaking.
I would also beware of alignments done at most shops that offer these $50 deals. Its easy to show changes in numbers on the computer, but unless everthing is set up correctly, things could still be way off. I would trust someone with experience and skill and no measurement tools before I would trust an unskilled "monkey" with a $100K alignment machine. Just something to bear in mind.