if you have to ask about the carrier and etc take it to someone.

tighting it some probably wont do anything for ya. if the nut did back off some then that means youre setup is probalby off now which now resulted into wear issues and etc

chances are the pinion seal is just leaking but you still have to tear into it. you could try and get away with the first thing stated but if you mess it up youre looking at good money into repairing your mess up