Shaking/jerking at high speeds

I noticed this morning that whenever I went over 55 mph, my 2000 Sunfire starts jerking lighting or shaking. The faster I go, the more apparent this shaking sensation is. Any ideas on what this could be? It was fine yesterday on the highway!
And by the way, my check engine light has been on for several weeks (I am getting it checked out soon!). The code comes up as P0440 "Evaporative Emission Control System Malfunction". Could this have something to do with the shaking?

I'm not sure about this, but you could look into the following and see if there is a problem.

I recently had the same problem 2 days ago. I couldn't go any higher than 40mph because the car started to shake and jerk around. I looked into the problem and found out that my rear shock/strut had a bad spring cover (the top plate that holds the spring). I would recommend to inspect your suspention first, if you don't find anything wrong with it try checking that your engine doesn't move out of place. By this I mean that it doesn't wobble easily. You could also make sure that your tires are put on tight (although I doubt the tires would be a problem. Maybe they would need to be balanced though.) These are just some suggestions; I hope they help you out.

Best of luck,
Insany

Or maybe your tires need balancing.