Changing Tail Shaft Bushing

I had the same symptoms in 1968 barracuda I bought that has a 727 in it. Trying to solve it, I put new tires (with balancing) on it and then replaced the tail shaft bushing but the problem continued. Vibration over about 40 mph. I had a new (slightly longer) drive shaft built using the old ends. Voila- problem solved. Apparently the slip joint did not have enough overlap. I also had a leak that did not go away but later I found the leak was actually coming from the neutral safety switch which had its seal gone bad.