to any auto techinician or shop it means a fluid change. If you have some sort of posi you will need an additive for the clutches. You would be safe by putting a synthetic 75w-90 in it. And if you have a posi there are several different additives. Silicone would work on the diff cover, but they also do make gasketswhat does diff service mean to you? To me it means getting a master rebuild kit and replacing the ring and pinion and all bearings/races/seals. Maybe it just means a fluid change to you?