vibration hopping bouncing in the rear end

I can drive up to 45 mph then this thumping bouncing happens in the rear. If i punch the gas it stops and a i continue to accelerate w/o any problems driving 70 mph on interstate no problems. It is just when I hit that 45 mark 5 10 15 20 30 40 50 60 70 no problem but I always have to punch it at 45 to stop the vibration and it not a minor vibration. It feel as though when i punch the gas one rear wheel is more powerful for that short people of time.