It's your axle breather tube and the most common reason why differential fluid would be coming out is because it was over filled
If this is something new it is worth trying to find an IR temperature gun and monitor different areas of the differential housing. the oil will expand more when it is hot. If a bearing is going bad it will get hotter.
Also, how long has your ARB locker been installed? If oil is not flowing back into the differential sufficiently past the locker slip ring it could cause the tube to become over full and starve the housing of oil. I recently learned this the very hard and expensive way.
Not too sure how long it's been in..it was in when I bought it. Should I take the diff cover off drain it and fill it? How would I fix the problem of it is starving the housing?
If it were an ARB install issue it would've showed up withing a few hundred miles..... Mine didn't make it to 100.