F350 rear differention warm/hot to touch

My trucks rear end gets very warm to touch after driving. When I touch it just the pumpkin is hot not the axle or break shoes or wheel rims. The pumpkin stays hotter than the exhaust after it's been parked for a while. The axle seals aren't leaking and the fluid level is OK. The fluid looks used but not used-up (it's slightly dirty, but there is no strange smell or taste to it). What do you think the problem could be?