With fresh fluids in the rest of the drivetrain, it is now time to turn our attention to the axle lubrication. Where as we stated that the transfer case fluid leads a relatively easy life, the same is not true of the rear axle. The reason for this is heat; due to the relatively high frictional losses of the hypoid ring-and-pinion gears, axle lube temperatures can exceed 300F during high-speed cruising. While certain ultra-high-performance vehicles such as the Corvette Z06 have provisions for axle cooling, most of us aren't so lucky. Throw in the fact that this lube isn't filtered and is prone to contamination by water and other environmental matter, and it's essential that it be changed on a regular basis (two years or 25,000 miles isn't a bad starting point for a recommended service interval). We'll show you how it's done on the 9.5" 14-bolt axle under the rear of our '96 GMC K2500.

Changing out the axle lube can be a bit messy for those vehicles that are not equipped with a drain plug, but fortunately the task only requires regular hand tools and a catch pan. We'll also need the required amount of a suitable lubricant (we used a synthetic blend of 85W140 gear lube), a new cover gasket, and some RTV silicone sealant. It's also possible to eliminate the gasket entirely and just use RTV or some other form-in-place gasket material, but we've had a long history of success using a gasket and we don't like to mess with something that works. Vehicles with a limited-slip differential may require a friction modifier, which can be purchased from a well-stocked parts store or your local dealership's parts department.

If your vehicle has a drain plug on the rear axle, it's a simple job that mostly resembles the transfer case fluid change, and you can skip most of the steps below. Frankly, we don't mind popping the cover off once in a while just to see if things are OK.

Due to the high viscosity of axle lube, we suggest running the vehicle around for a few minutes to warm up the axle before getting started.

Before we get too far into this project, it's a good idea to break loose the fill plug, which is normally located on the "nose" of the differential. We start here because it's a real bummer to go through the process of removing and replacing the cover only to find out that the plug is frozen or rounded-off and needs to be drilled out. Once you've verified that it can be removed, feel free to move on to the next step. If you find that it's stuck and you are not familiar with the various methods of removing frozen fasteners, you still have a vehicle that can be driven to a repair shop for professional assistance.

Next, we move around to the rear of the axle and loosen the cover bolts. Don't remove any of them at this time. Feel free to keep comments about the level of rust to yourself, as I think I'm quite aware of the fact that 10 winters in the Salt Belt have taken their toll.

Along the way, you'll likely encounter support brackets for brake lines or ABS sensor wiring. Use caution when moving these components out of the way, but make sure that they're clear of the diff cover.

Once all the fasteners have been loosened, go around and remove all but the top bolt. This bolt should be backed out until only 3 threads or so are still engaged; the goal here is to allow the cover to hang from it instead of dropping into the drain pan.

Using a suitable instrument, gently break the cover free from the axle housing.

The lube will come from several locations around the perimeter of the cover, so make sure that the catch pan is properly positioned or else you'll have a mess. If this is your first time encountering used axle lube, take a few minutes to appreciate its unique aroma.

Once the majority of the fluid has been drained from the axle, remove the last remaining bolt and set the diff cover aside for a moment.

Judging by the build-up of sludge inside the axle, we let this diff lube go far too long between service intervals (we won't admit to how long we think it's been since it was last changed). Ideally, we'd see no build-up or deposits of any sort inside the axle housing; there should only be a clean oily film on the differential components.

Take a clean rag and wipe out the bottom of the axle housing. This particular axle has a magnet placed in the housing, so make sure that any ferrous material is cleaned from it (other vehicles will have a magnet located on the inside of the diff cover). A deposit of fine silt-like material on the magnet is perfectly normal - remember that there's no filtration of the fluid - but chunks are bad (and trust us, shrapnel poking through the diff cover is worse).

Using a gasket scraper, gently clean the sealing surface of the axle housing. Be careful not to damage the surface (in this case, we're dealing with cast iron so it's pretty difficult to hurt it). We also performed the same task on the diff cover.

After a quick scrub-down with a Scotchbrite pad and some solvent (as usual, we turned to brake cleaner to get the job done), we're ready to begin reassembly.

A thin film of RTV is applied to the axle housing. There's absolutely no reason to go overboard and apply a thick bead, because it'll just squeeze out anyways when the cover is tightened.

As stated previously, we elected to use a gasket. Make sure to know your exact application when purchasing a gasket; for GM's full-size truck line-up in '96, there were three different axles, and of course they each use a different gasket.

Align the gasket with the axle housing and use the RTV to hold it in place.

A thin film of RTV is applied to the sealing surface of the cover, and it's set into place. The bolts are reinstalled, and we torque them to the correct value (30 ft-lbs for this application). To be honest, we've previously had success in tightening the bolts without the use of a torque wrench, but that's definitely not the "right way" to do this task. Spend a few hundred dollars on a good torque wrench, and it'll last the rest of your life if cared for properly.

Now it's time for the tough and slow part of the job. We remove the fill plug and begin the process of filling the axle. Due to the viscosity of axle lube, it may not be a bad idea to soak the bottles in a bucket of warm tap water to make the thick liquid flow a bit easier. Depending on the amount of clearance above the fill plug, there may not be room for the quart-sized bottles. If that is the case, we attach a piece of hose to the bottle's nozzle. On this particular vehicle (a lifted truck), we had no lack of room in which to work. If a friction modifier is required, we add it to the axle before adding the lube so that we don't forget to do it later.

The diff is full when lube begins to overflow from the fill plug. In this case, it took just shy of three quarts to top off the diff. Once the overflow slows to a trickle, reinsert the fill plug (it may not be a bad idea to add some Teflon pipe tape to the threads to ease future removal), and torque it down to the proper spec (here, it was 20 ft-lbs). Hint: if there is any sign of damage whatsoever to the fill plug, do yourself a favor and locate a new part to install into the diff. As we've learned the hard way, reinstalling damaged fasteners is stupid.

The used axle lube can typically be recycled along with your motor oil, but please verify this with your local recycling center before potentially contaminating a large amount of waste product.