Thanks for information. How bad is sagging? Why is it a common problem? What's causing it? If I don't do anything about it, would it harm the car? What can I do to fix this problem?

I am thinking that maybe the tires are the causes for shaking at higher speed. (they are sumitomo tires.. anyone has any experience on these? they are not due for a change, but i will be sure to get michelins which i've had good experiences with.)