[62stepside longbox]

Here it is in simple form:
To replace the seal you will just need to remove the yoke. This will mess with the pinion bearing preload but it's pretty easy to get back into spec.
But to replace the bearing you will need to remove the entire carrier and pinion and use a press to get the old bearing off and put the new one on, this will mess up the pinion bearing preload and the gear spacing (pinion depth).