I had it on my '04 and it did make a bit more intake noise than stock. It seemed to have a bit better throttle response for sure. It won't help for mileage though. I would suggest putting a screen in there if you decide to do it.
Got a lot of mileage out of that $10 carpet. Also lined the bedside cubbies and ammo cans and built this concealing box. Still have enough for a work mat for the tool box.
It's called a seal. :)
I think this is what you are looking for. I had mine replaced a while ago also.
http://www.toyotapartszone.com/oem/toyota~seal~oil~for~front~differential~case~90311-47012.html
Most folks get them from here:
http://downsouthmotorsports.com/i-135658-toyota-tacoma-front-bilstein-5100-ride-height-adjustable-shocks-05-2014-w-optional-rear-shocks.html
If you are comfortable using a spring compressor then you could do them yourself. Just remember to get an alignment when...
You can go up to a 265 75 16 on stock suspension. If you want a mild lift I would suggest skipping the leveling kit and get some adjustable bilstein 5100's. Most use the 1.75" setting to level the truck and the ride will be much better than a spacer lift.