leaking diff fliud from passenger rear wheel

this morning as i left i noticed that on my project truck there was fluid running down the inside of the tire coming out of the drum cover. at first i thought it was a brake cylinder in the drum but its diff fluid not brake fluid.
the truck is a 2000 dodge ram 2500 v10 4x4 the rear axle is a full float dana 70 if any one can give me some insight on whats causing this and how do i fix
thanks