2004 Pontiac Grand Prix shaking really bad I need help?

[mom of 1]

I was feeling a lot of shaking coming from my car every time I hit the accelerator and made it to speeds of 40 - 45 mph that's when it got really bad so I thought it was my tires going bad, so I went and got all new tires put on thinking this will fix the problem. Well now that I have all new tires on the shaking is still their but worse.every time I hit the gas it shakes really bad and still sounds like as if my tires are "jumping" the whole car will shake no matter what the speed is until I take my foot off the gas then it just vibrates can someone please tell me what this could be..thanks you