If it was leaking before, its probably worth replacing while you have the driveshaft out. That seal is notorious for leaks. The seal will be ~$10, NAPA and Carquest are likely the only chain stores that will have any seals in stock.
Oil should pour out of the t-case when you pull the driveshaft.
On the back side of the case, there are two plugs: one to fill and one to drain. Remove the one that's higher up and fill the t-case until fluid comes out of the hole.