1995 Ford Mustang Gt Tranny or Clutch Problem?

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Hi, about three weeks ago I was going down the interstate and I noticed that my shifter was vibrating and I heard a slight rower coming from the transmission. It soon went away. Two weeks ago I drove to the store and I got on it a little in 3rd 4th and 5th. After that it made the sound again with the vibrating in the shifter. Well Saturday my wife drove the car about 4 miles going down hill and she says that the back wheels locked up causing her to fishtail and loose control. I figured that the rear end had went bad. I pulled the back plate everything was fine its a 8.8 327geared rear end out of a 97 Cobra.I drove it and it was pulling and going into gear just fine. I let off the gas and all hell broke loose, It sounded like the whole transmission was coming out. I hit the gas and the sound went away. Any clues? Transmission or clutch? I am lost and don't know what to pull.