When I ease into the accelerator and hit the top of 3rd and bottom of 4th right around 45 mph the car vibrates really bad. It's done this for about 10 or so months now. It only does it when I ease into the gas and it goes away if I give it gas as it's vibrating. Do I have a warped driveshaft or something? Anybody know?

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1986 Berlinetta, 2.8L, newly rebuilt 700R4, 3.42 Posi. 127,000 miles and counting.
I've come to realize that I'm gonna have this car till it pukes for good, b/c I'll never be able to bring myself to sell it.