A bad u-joint will act like that sometimes, to make sure it is not a eng miss, hold the brake pedal and apply throttle litely and graduly work to hard throttle, remember turm off traction control and don't do a brake stand, if you can't get a miss look at the u-joint. I had a vibration on decel around 80km to 50km (50-30 mph) and was the rear end crown and pinion, changed out at 1800 km. Just food for thought.