So I have a 2005 Tribute v6. It has a hard vibration like the tires are out of balance but only when I hit the gas at above 60. Smooth as glass when I let off of gas. So its not the tires. Even at 80 its smooth unless I hit the gas.

Any ideas what could cause this shaking?

 

Check all the driveline couplings, likely one has some movement under drive which moves it out of balance but it relaxes under over-run back into its correct balanced position.

If they are all good Id suspect one or more soft engine mounts.

 

had same problem. had Mazda do a check and they said my right front axle was loose. 2003 v6 ES