You should see two breathers clipped onto the the inside of the driver-side fender and, from what I remember, one is for the front diff and the other is for the tranny/t-case. I put my rear breather up in the jack storage cubby.
You might want to go through this thread... http://www.toyota-4runner.org/off-roading-home-monst4rs/123-breather-install.html My posts are on page 7 (#100 and #102). One thing I'll add is that on a road trip from Colorado to Indiana in the middle of the summer running AC with "recirc" on, I noticed the faint smell of gear oil. Just clicking "recirc" off every so often cleared it out instantly. That was the only time I've ever smelled it though.