I had one of Superlift's new eXtreme Rock Rings to install over the differential cover on my Jeep and figured now would also be a good time to change the gear lube. As I was doing it, I thought, just how many people really do this the right way?
So, for whatever it's worth, here is my take on this job. No, I'm not going to go through all the safety warnings. If you can't figure out how to do it safely and environmentally, then stop reading right now. However, it's a pretty good idea to keep the stuff away from your skin.
The fluid is going to drain out a lot faster, more completely, and carry more suspended contaminants if it is warm. I'm a pretty sloppy guy, so I use a big oil-catching pan under my catch container. Remove all the bolts except the top two, but loosen these up. Carefully pry the bottom of the cover away from the bottom of the case. It may be tough to get free and require a bit of work, depending on how stuck the gasket/sealer is. You leave the top bolts in, so the cover doesn't fall off and knock over your container. How far open you pry the cover determines the flow rate.
I like to catch all the lube in a clean container and then run it through a paper paint filter. What's trapped in the filter gives me a pretty good idea of the gears' and bearings' conditions. Don't be too alarmed when you see micro-sized shiny stuff. It's just the result of normal wear. What you're looking for are flakes and pieces you can actually feel with your fingers. These are the indicators of something dramatically wrong.
Once the majority of the lube has flowed out, you can remove the cover completely. On a rearend where there are no axle seals at the differential housing, I like to jack up one side of the vehicle and then the other. This lets any lube that's in the axletubes also drain out.
While the residual lube is draining out, take off the cover plate and start cleaning it up. Once all the grease and grime are off, start on the gasket. It may be a real fiber gasket or it may be some type of RTV sealant. Either way, get it all off. The next step is what most people don't do, but I feel it's really important. Run a flat coarse file over the sealing surface, both on the housing and the cover plate. This makes sure that there are no high marks that may interfere with proper sealing, and the rough texture from the file gives the gasket or sealer a place to bite into. You may find that someone in the past has overtightened the cover plate and made a raised area around the bolt holes. Find a flat surface and pound these back down. Make sure the cover plate lays flat on an even surface. A piece of window glass works for me as a surface plate.
Take a look at the condition of the teeth on the ring gear. The pinion-gear teeth are harder to see, but the ring-gear teeth are readily visible. Rotate the gears around so you can see them all. You're looking for abnormal wear, as the gear's contact surface should be clean and shiny with no bluing, ribs, chips, or pits. Make sure that you clean out the bottom of the housing as usually there is a little pocket that can collect junk.
Once you're satisfied that everything is looking like it should, put on a new gasket and install the cover plate. While I like to use a real gasket, others prefer a sealer like The Right Stuff or one of the RTV sealers. If you choose to do it with the latter, here is a little tip I try to use if time is available. Apply the sealer and lightly tighten the cover plate bolts, just enough to slightly squeeze out the sealer on all sides. Now let it sit for 24 hours before going back and tightening the cover down completely. Yes, there are special torque specs, but I have never seen anyone ever put a torque wrench to a differential cover. Just snug is tight enough. What you're doing is actually making a gasket, not just a seal.
Important step here: Don't forget to refill it! Put a big piece of masking tape across the side window that says "Fill with lube," or some thing like that, and don't remove it until you've filled it. See it wasn't that hard, was it?