My car shakes violently once in a while when its in overdrive and i hit about 50-55. What could be the problem with it

Torque convertor lockup shudder comes to mind...very common, usually goes bad around 75,000-90,000 miles...

Does it go away if you gas it hard or let off the gas?

Does it go away if you gas it hard or let off the gas?

That is exactly it. So how could i get this fixed??

If that's what it is (get a tranny shop opinion,etc) there is a solenoid in the trans that has probably gone bad...it is replaced through the vertical pan on the driver's side of the tranny...the tranny/motor has to be jacked to get access, kind of a PITA...

90% of the time that's the problem...there are other ways the mechanism can fail, but shudder is usually the solenoid chattering and making the lockup do the same...

Tire out of balance.

That happened to my car. I could drive fine up to 45 mph. Anything above that and it would shake so much I had to slow down. Changed tires and I was set to go.

Yeah most tire defects and balance issues show themselves between 50-60 mph, but won't go away the instant you step down on the throttle or let off...and he said it only does it from time to time...a balance/defect issue would be consistant...