I was rooting around my engine compartment, giving it a once-over and noticed this random hose beneath the air intake box. It appears to be some sort of plugged hose.
It looks like it goes down to the front transfer case, but I have no idea what it could be. Should this be in use? What is it for?

 

Thats the vent line for your front differential...

believe it or not that is not a plug, its just a cap that prevents dirt/debris from entering the hose.

If you have an auto trans you should see one zip tied to your trans dip stick that looks just like it... Its the vent for the tcase/trans.

 

^^^ he nailed it.

 

Also one on the rearend that attaches to the frame rail

 

Funny you mention that - I just noticed it right after I took those photos...

Thanks for your help. Are those usually secured somewhere, like the one by the dipstick?

 

On my first gen the front diff attaches to the inner fender behind the drivers headlight. The trans and t-case attach to the firewall beside the trans dipstick. The rear I explained. Only other vent I can remember is fuel and that's on the filler neck.

 

There's a small hole in the upper fan shroud by the rad hose that it should clip into... as long as the clip isnt broke that is.

 

This is true... but it might be a little harder to find that one while giving the engine compartment a once over.

 

If you can find anything on the rear framerails via the engine compartment I needed ya eariler so I wouldn't had to take off the power steering to change the fuel filter in my noma.