If tires are ok, and none of the U-joints are bad, and the drive shaft has not lost any of the balancing wheights and has not been disasembeld and turned out of balance, check the shock absorbers some times one or both shocks on the rear or front will be bad just enough, and all it takes is a small bump on the road to set a vibration going.
To check this out, drive until the vibration starts, note the speed and then slow down, do not stomp on the brake, note when the vibration stops, if it stops fairlly soon then the shocks are bad if it stop a long way down the decel curve then you have someting else, maybe a torque converter that lost a balancing weight or a bad bearing on the rear axel.
navegator