Marlin Crawler's site has a great writeup with pictures on how to tear down the t-case. The crawler gear install instructions show how to tear down the entire thing. They rarely go bad though, so you probably just need a seal and/or a nut re-torque. Unless you bent the output or something.

The pinion play would have to do the differential. If its a stocker, its probably far cheaper and easier to find a used one, but there is also some differential setup info. As Roger said, the rear wheel bearing and seal replacement procedure is also here on this site.