Yeah you've got all sort of sealing at the ball at the back of trhe transmission. You have a cork seal in the retainer for the ball itself, another seal that screws on to the end of the ball where it passes over the torque tube and also as you said the okie bushing and seal inside the torque tube.
Usually if it is the okie seal, the oil level keeps increasing in the differential, then it get over full.
John
