I noticed a spot of gear oil under the rear differential of my 1977 Jimmy. The number and size of spots grew over about 20 miles of stop and go driving to the point that gear oil had blown all over the exhaust system and bottom of the truck. The vent in the differential has a rubber hose attached to it that is about 18 inches long and looks to be torn on the other end although I have been unable to identify where it was once connected. It is from this vent hose that the oil is blowing out. The differential is a 12 bolt with the gov lock or after market posi (can't tell which) and 3.73:1 ratio gears. I pulled the access cover and there was oil about 3" deep left in the rear end. Nothing looks to be broken and the rear end doesn’t make noise. The gear oil did have a look like silver paint or fine aluminum powder had been added to it indicating something worn? I've never heard of this problem before and can't think of what would cause pressure inside the differential sufficient to cause the gear oil to be forced out.