[Ggmac]

Is it a vibration only in overdrive? How much u-joint yoke is hanging out of the trans ?
When od automatics first became common place , it was very important to have a good bushing in the trans tail shaft . Any slop or excessive driveshaft yoke protrusion would create a vibration , usually worse in od . If you can accelerate past the current vibrating speed in a lower gear then shift into od and it starts vibrating , it'll continue to vibrate till almost stopped .
Good luck