I cant go fast on highway because I dont know if it's dangerous... it keeps shaking... it's an 850 GLT ... I seen someone talk about economy mode or sport mode.. .does that have anything to do with it?

 

I don't think that ECON and or SPORT mode will have anything to do with shaking at high rates of speed. It is probably something to do with your suspension and engine mounts can be a culprit as well.

 

First thing to check is your tires. If they are slightly missbalanced you'll feel it at higher speeds. If they are hugely out of balance you'll feel it at lower speeds and all the way up.

My truck shakes at 55-60 and I know it's cause of my tires. If I were you take them in to bell tire or some other tire store and have them rotated and balanced. There is a really good bet that that is your issue.

 

Also make sure one of the tires didn't Break a Belt in the tire.
Also have the Tie rods inspected and the Balljoints.

 

thanks guys I wouldnt be surprised if it has to do with the tires not balanced..

 

Could be a bent wheel too, but the tire place will tell you that when you have them rotated and balanced.

Funny story: My brother in-law and ex Faifax County police officer was test driving a car I found in the DC area for me. He called me on his cell phone while in the car. He said he felt shimmying around 70-75 mph. I said take it up to 100 and see if it goes away. He laughed like I was telling a joke or something. When I went and bought it, the 98 V70 it does have a little shake at 70 to 75. I'm pretty sure it's the tires, and I have not changed them or had them balanced and rotated yet. I hardly ever drive the car and my wife never complains about it.