Auto Parts: Car Vibration, control arm bushings, 2003 hyundai sonata

Question
I have a 2003 Hyundai Sonata. The car has been vibrating pretty badly at the point when it's crossing 100km/hr and above. If I raise my foot off the accelerator at that speed the vibration goes away...but once I mash the accelerator the entire front shakes violently.

Initially I thought it was an engine mount issue, so the rear and left mounts were replaced. Since I have done that it seems that the vibration has gotten worse.

What could be causing this?

Answer
    I notice that you didn't replace the front mount.  If I had to pick one of those mounts that might cause the problem you described, it would be the front mount.  It is the one that is intended to absorb the torque of the engine.  I would also give a hard look at the health of the right-side CV axle assembly.  These very long axles can act very strangely, especially when there is a little wear in the joints.  There may not be enough wear to give the normal CV failure signs, but it doesn't take much to get these long bars of metal vibrating.  Ford used a large rubber donut mounted halfway down the axle shaft to damp these vibrations; other manufacturers have done the same, but not Hyundai.  Any amount of play in the strut mount or the lower control arm bushings could allow a slight wheel/tire imbalance to be amplified under acceleration (or deceleration).  The first place to start is making sure that the tires are impeccably balanced.  If you can find someone who can balance the whole assembly on the car, do that.  Then I would get a front end shop to really go over the suspension and steering to make sure it's tight and healthy.  And I would go ahead and change that front mount, especially since you changed the other two.  It's the cheapest and easiest to change, and the most likely to cause problems.