I've had it cracked open while replacing one on my 2004. What does it mean though to rebuild one of these? What gets replaced or serviced?

 

Generally they get rebuilt due to the ring and pinion gears failing. Also what gets replaced when re-gearing.

 

If you do quite a bit of offroading, look into changing from the 2-pin to 4-pin.... as well as pegging it:

https://www.ashcroft-transmissions.c...ff-centre.html

https://www.ashcroft-transmissions.c...gging-kit.html

 

The 2004 is dead quiet, but my 2002 has almost what sounds like a supercharger whine at highway speeds. Is this an instance where rebuilding the diff my be needed? I guess I should expand the question to what it looks like to rebuild the transfer case too...

 

@Brandon318 drain the transfer case and both diffs and refill them with good quality gear oil, a little heavier weight than stock. Your whine may well go away, I use a 75/140 Amsoil synthetic and my Disco has no whine

 

Just hit up Goldwing Rovers and get a used diff, they rarely fail unless you're doing trails.