While driving down the highway yesterday at about 55 mph I noticed if I quickly depressed the gas pedal that the rear of the car seemed to drift to the left. Almost as if the left rear broke loose but I didn't jump on it that hard. It almost feels like the axle or entire rearend slid a little to the left like it wasn't attached to the car. If I slowly roll on the gas it doesn't happen.I came home tonight and pulled the wheels. The control arms are secure. The bushings all seem fine and the axles have very little in and out play. I checked the play on my 69 and it was about the same. It is a12 bolt non-posi rear with 3:73 gearing. Any clues?