I have been trying to find the source of a severe vibration in the drivetrain of my 65 Mustang which occurs at 60-70mph. After going through a process of elimination and not finding anything, I decided to pull the driveshaft from the C4 automatic. Just for kicks, I grabbed ahold of the splined output shaft sticking out of the tail housing and it actually wiggled back and forth about a 1/4 inch play either way. Is this normal? If not, what does this mean? Could this be the cause of the mysterious vibration I am getting? I have eliminated everything else...tires, front and rear wheel bearings, etc... The vibration does not occur when revving the engine in park, only when it is in drive. I even pulled the rear axles, put her up on jackstands, and let her rip up to 60mph and the vibration was still there. I also pulled the third member and it seems fine too. Has to be in the transmission or driveshaft.