between 35-45 and then again above 75 MPH, but only when I'm on the gas and accelerating usually. I rotated the tires, changed the upper control arms on both sides. One control arm was bad, that's what I thought cause the shaking. I checked all the ball joints and the bushings in the front all seem fine. It seems to shake in the front thats' why I didn't check the back, now that I think about it.

Does any one know what causes this?

I should add that this shaking did not come about after I took the wheels off. The wheels were on the whole time since I bought the car months ago.