i think lee jumped the gun here. let me start of by saying this....if you don't know what a rear diff is, then you don't need to extend it.
here is the run down. the rear diff is also refered to as a pumpkin. it is the big round piece between your rear tires. it has gears in it that are turned by the driveshaft coming from your tranny/transfer case. it has gear oil in. makes your tires go round and round.
i am guessing you are talking about extending the diff breather. due to heat caused by the friction of the gears as you drive, pressure builds up in the diff. this pressure needs someplace to go and WILL find it's way out. whether that be by blowing seals and letting oil leak out or whatever. what toyota did is put a breather on the top of the diff. it's a floating cap that opens and closes based on the pressure under it. easy enough to follow, right?
the problem is that when wheeling, this breather can open up and let water in if in a deep puddle or water crossing. this in turns requires a drain and refill of the gear fluid inside the diff as oil and water don't mix. what the extension mod does is uses rubber hose to extend the breather setup to a point where it will never be under water. most people put it up to the gas fill door.