There's a hose the looks like that in the engine bay as well

 

Yup looks like a vent line to me also, either for the brakes or rear diff like another member said(sorry spaced your name.. its early.. lol) make sure u put it high as you can when you tie it off so that way water doesn't get in it and create issues.

Josh

 

Mine has a clip on it and attaches to the top of the drivers side shock mount (on the sheet metal).

I always laugh when I see trucks dragging these tubes down the road.

 

It has nothing to do with the brakes. Brakes are sealed and work off of pressure. Just because it's part of the brake distro block doesn't mean it's for the brakes. it has to do with assembly line costs most likely. It's just a vent tube that raises the vent higher than where it would be submersed in water if you were to back into water or drive through water.
There is one up front as mention for the guys that have 4wd because there's another differential up there. duh.
I think you guys are just getting confused since it appears to go with the brakes. it doesn't.

 

Alright well when I get home I'll tuck it up higher than I did the other day. I just tucked it in on the frame for the time being but if it need to go higher by the top of the shock mount then thats where I'll stick it. Thanks for the info guys

 

well venting whatever... lets all just tuck 'em under and smile

 

X3 on the vent to the diff. That isn't a plug on the hose end, it's a round-about filter to keep water, dust when the axle goes submerg.