Interesting....I've heard about this and met a guy from TNX who had his done and the breather was in a tail-light.
Is this really that needed?
There are two problems people have with the stock breather:
1. Water can easily get in there if the axle is submerged, since there is no hose on it like the front diff.
2. The valve can get clogged up and pressure or vacuum can build up inside the axle causing the seals to leak.
If you don't do too many water crossings, and check your valve regularly to make sure it's not clogged or stuck, then you shouldn't have too much of a problem. Doing the mod is real easy though, you just screw in a different connector and run a rubber hose up somewhere higher (doesn't have to be behind the tail light).