The rear differential cover (Mopar 8 1/4) and part of the differential tubes are covered in a thick, hard black sludge. So before removing the fill plug to check the gear oil, I decided to scrape at least some of the crud off (don't know where it came from). In doing so, I spotted a small rubber tube just hanging there in the air (photo #1). I followed it over to the driver's side where it goes right into the frame rail above the small leaf spring "bumper" if I'm not mistaken (photo #2). Any forum member have an idea what its purpose is and if the open end should be attached anywhere? Than you.

 

Vent tube for rear axle. Should be attached to the top of that brake line fitting in the top of the axle tube.

 

I agree you should also change the diff oil seeing as the vent has been open to all that dirt seen on the axle tube.

 

The sludge is gear oil mixed with road grime that was sloshing out of the axle tube where the tube should have been connected.