Put a 2.74 gear in it and you won't reach the MPH necessary to cause vibration . I agree with many other that it sounds like pinion angle to me, Do as suggested and check to make sure the spring perchs are good and not cracked form housing. Could still be a balance problem with the engine somehow. If the B+M flexplate is supposed to be balanced for 360 and the converter people no the converter is going in a 360. The converter may not be neutral balanced and could also be balanced for 360. I would think if both these items were balanced for 360 you would get an out of balance effect.
. I agree with many other that it sounds like pinion angle to me, Do as suggested and check to make sure the spring perchs are good and not cracked form housing. Could still be a balance problem with the engine somehow. If the B+M flexplate is supposed to be balanced for 360 and the converter people no the converter is going in a 360. The converter may not be neutral balanced and could also be balanced for 360. 
I would think if both these items were balanced for 360 you would get an out of balance effect.
