vibration problem

no problem sitting still or neutral. replaced the flexplate when I installed the convertor. The bushing in tailshaft sounds like it might be bad. The vibration feels like it is driveline related but it has been balanced.I feel it more around 45-50 mph and I feel it in the floor and some in the wheel.feel it most when I accellerate and let off the gas.It feels somewhat like a grind instead of a bounce type vibration. I am puzzled.