After running my Big Bear 400 4x4 hard, I noticed a small leak of what smells like Hypoid gear oil beneath the tube that runs from the rear (final) differential. Is this some sort of vent tube?

There is no oil leaking around the differential housing itself.

 

The tube coming off the top of the differential that has the 90 in it at the top of diff is a vent tube...not really sure how fluid would get out of it though unless it isnt up on the quad somewhere to serve as a vent and not allow water/junk back in it! I have this exact quad of which you are speaking and with 2200+ miles I have never seen fluid come out of the vent tube. Check your fluid level in the differential and make sure that it isnt over full and make sure it isnt all milky like water is mixed in! Report back with any findings and further questions!

 

I had a 98 big bear and i rode it hard one day and i had oil from both the front tube and back tube pissing out oil and they were both vented up very high.. I actually had this happen to me last summer with a 350 rancher 2x4 i was ridding pretty hard during a pretty hot day and when i stopped my engine was covered with oil i almost **** but then realized the vent line was right above then engine and it just blew out a little.

 

Yeah, I talked to someone else, and they had the same experience. I checked the oil level in the diff and it was fine. I also checked the vent hose routing, and it is high on the quad as it should be.

I'll just keep an eye on it to make sure it isn't anything major.