put a mark on the center & the nut , then undo the nut , you will need an impact to do this , it is torque to 250 ft lbs , once the nut is removed use a hammer & tap the yolk off , now pry out the seal & replace ti , check the yolk to see if it has a bad groove where the seal rides , you can fix this with a sleeve if it is worn , now reinstall the yolk & retighten the nut until the marks you had made line up again , top up fliud level