front end shakes

ok...i went from west virginia to virginia today and all of a suden about a hour into the trip my car started having a problem..like it shaking real bad going down the road like the wheels in the front are gonna fall off..but the crazy thing is that it only shakes when your on the gas..if you let off the gas the car just floats like a brand new car..and the rmp gauge is bouncing up and down real bad..as well as the speed gauge..but if you hit the gas hard and the trans down shifts to 3rd to get around someone, the car will stop shaking but soon as it goes back to 4th the car is acting like the wheels want to fall off again..the car was shaking like that the last hour of the trip.and i have a big power lose..what could be my problem?? i am lucky i even made it home..thanks!!