vibration problem

I'll add my name to the list of many with this problem. Mine gets into a harmonic at about 67 mph and the pulsation then increases with speed. I don't want to push it past 75 mph to see if goes away at higher speed. Does nayone know how much radial play there should be in the tail shaft of the A727? Since O'reilly apparently sold me the wrong rear seal (they're battin' about .150 overall), I have to pull the drive line again anyway so I'd like to check it.