as stated above your differential is set up like any and all rear wheel drive vehicle . your diff is kinda sealed ,it has a breather and it uses 90-140 gear oil, it is a open rear . but use a good oil, your oil should be black, but not sludge. most of the time the rear gear oil will last 10 to 15 years well over 100,000 miles ( if u have a wave-trac limited slip unit then order oil from needswings . only a few oils work with the wave-trac.
take it to a shop , and ask them to check the differential oil and you want to see them do it because you were told it is full of sludge they will let u see it done.