Car Shaking

I have a 2001 sebring convertable that the body shakes at around 45-50 mph. I have replaced all 4 tires balanced all 4 tires changed the front rotors and pads replaced the right rear ball joint that needed replacing and it still does it. The weirdest part of it all is that it does the vibrating most in the morning and not as much later in the day. Any suggestions will greatly be appreciated. Thanks.

try the basics fuel treatment air filter then go from there it my be something in the motor leaking when its cold and sealing when its hot its impossible to tell so just do the cheap stuff first and work from there