I noticed that my rear springs are missing the top cap on one side...

how important are they for a smoother suspension?

 

and what are their purpose in life?

 

I believe it's to make the ride smoother, but like half the stuff GM did to these cars to "improve" them, they really served little to know purpose.

 

Could it be the cause of my car swinging from side to side over 80mph? Im missing the right rear, and it swings really bad...

 

I heard a few people say their cars swing or "fishtail" really badly above 80, or so. I have not noticed that in my car so I can't give an honest answer, but I'd say no. I think it'd be related more to tires.

 

Are you saying that you're missing an isolator on your right rear spring but you have one on the other side? Your rear will be leaning if your missing an isolator on one side.
If that's the case, you need to put an isolator in there obviously. The part is not that expensive so you can pick one up at the dealer.
Do you have any pictures of your car that we can look at?

 

The isolator is there so you don't have metal to metal contact. If you keep riding like that your spring could very easily rub a hole in your underbody.

This could cause the sway. Mine swayed for a while. It was a loose LCA bolt.

 

Im tightening up my back a.s.a.p!

As for the isolators...Ill be buying a new one soon too!

thanks guys!