+1 to what the general said.
I think
@Rdub6 has it backwards. The tailshaft yoke slides fore and aft a bit to make up for geometry changes as the differential moves up and down. Remember A squared plus B squared = C squared? A is from the tail shaft to the vertical position of the pinion shaft. B is the distance from that line parallel to the body down to the pinion shaft center. C is the length of the driveshaft. Although B changes through an arc with wheel movement, it does change in length as the
suspension moves. So C also changes and something has to make up the difference in length. Driveshaft length also changes under hard acceleration and braking. A torque arm
suspension helps this by making the
suspension travel arc is one of a longer circle where C (driveshaft length) is the radius or close to it, but I doubt you have one.
But side to side or up and down movement indicate a worn tailshaft and or bushing. Besides eventual vibrations you will notice, the seal will not last long.
Now if there is a ridge on the yoke from where it most often hits the seal, that ridge will not be good on the seal every time you go over a bump.
