Looks like a rear diff breather tube.
This must be the 5 thousandth time someone has asked about this - its a common occurrence obviously. And I was one of the 5,000 right after I got my Jeep - I guess it doesn't like to stay zip tied where it belongs!
I have no idea why mine was loose - it was hanging down when I got home from the dealership.It must have came loose when I pulled out the inner fender liners