It is a breather tube I believe for the Transmission. You have one for each differential, the transfer box, and the transmission.

 

That is a breather tube. Correct.
Make sure it is tied up high and not hitting manifolds or Y pipe.

Congratulations on your HG job completed!

now replace the radiator, T-stat with a 180 and fan clutch and you'll be set.

 

the tube is in it's original location on the that firewall bracket. I did put in a new water pump and 180 stat, but didn't do the radiator or clutch yet.