rear transmission shaft

From the transmission seal to the yoke is about 1 3/4" to 2". I have a 904 which I compared to the 727 and they both seem to have the same amount of play in the tail shaft it's not excessive but I can move them. I am worried about the rear seal since I have gone through 2 of them now. I am going to pick up the drive shaft today and find out what condition it's in. I am hoping that is the cause of my vibration