Had To Do A Quick Repair On My F150 

That is an axle vent hose, it is there to relieve the pressure from the differential and the hose clips up high under the bed so water does not get in there, it has a check valve that lets whatever fluid rises out with the pressure return back into the axle, when these hoses crack or break it allows the gear oil to seep out..

Good idea to give it a quick spray down to clean up the area. That way any new fluid won’t be mistaken for an old problem. ✌️