Gear oil in the transfer case and the differential of your vehicle is like any other petroleum-based product. After time, just like the engine oil, it breaks down and loses viscosity. Because it's a thicker substance than engine oil, it requires less maintenance, but it should still be changed at regular maintenance schedule recommendations. Different vehicles use different weight gear lube and in some cases the transfer case may use automatic transmission fluid, even if the vehicle has a standard transmission.
Drive the front of your vehicle up onto car ramps.
Lift the back of the vehicle with a floor jack and support it on jack stands. This will elevate the entire vehicle, which will level it for draining the gear oil and optimize space underneath.
Put on the safety glasses and latex gloves and crawl under the vehicle (on a creeper if you have one) with all of the tools and replacement oil.
Locate the differential. If it's a front-wheel drive vehicle, the differential will be located on the front axle. All-wheel drive and four-wheel drive vehicles will have a differential in each axle, and rear-wheel drive vehicles will have a differential in the rear axle.
Inspect the differential for drain plugs (located at the bottom of the differential). If one is present it will have a drain plug with a square drive that will fit a 3/8 inch drive or a 1/2 inch drive ratchet. Place the drain bucket under the drain plug and remove the drain plug with the appropriate tool, turning it counterclockwise. Be careful not to let the fluid spill on you. Allow at least five minutes to drain. Clean any metal filings from the plugs with carb or brake clean spray and replace any copper washers that may be present. If not, wrap some thread seal around the threads of the plug before replacing it. Replace the drain plug snugly (not so tight that you squeeze the copper washer out of shape) once the fluid has drained thoroughly. If you have a drain plug, proceed to step 9. If you do not have a drain plug, skip this step and proceed to step 6.
Remove the bolts of the differential cover using a ratchet and socket. Place the drain bucket beneath the cover. Pry the cover off the differential using the screwdriver. Aim the drain bucket correctly because the oil is going to purge out of the rear end quickly and messily.
Scrape the mating side of the cover and differential with the gasket scraper to remove the old gasket seal and gasket (if applicable). Clean the surfaces thoroughly. Spray carb or brake clean onto the inside of the cover, wash out the old fluid and clean the edge surfaces. Wipe dry with the shop rag. Allow the rear end to drain thoroughly.
Place RTV silicone gasket seal around the edge of the differential cover and allow proper time to cure (refer to the directions on the tube). Place the gasket on top of the silicone (if applicable; some vehicles may simply use the RTV silicone gasket sealer). Apply the differential cover back onto the rear end and replace the bolts. Tighten the bolts alternately using the ratchet and a socket.
Locate the fill plug on the differential. This could be located on the differential cover as a rubber plug, or it could be located on the side (but on top of the actual axle) and require a square drive tool to remove. Refer to the owner's manual if you have a hard time locating it.
Clean the fill plug with carb or brake clean spray and wrap thread seal around the threads. Set it aside.
Place the hand pump into the bottle of gear lube. Place the tube of the hand pump into the differential fill hole and pump the oil until the fluid is level to the fill hole. Do not overfill. Refer to the owner's manual for the correct amount (especially if the fill hole is located on top of the axle). Once the proper amount of fluid has been achieved, replace the fill plug and tighten (or pop the rubber plug back on; whichever procedure is applicable).
Repeat the procedure for the other differential for all-wheel-drive and four-wheel-drive vehicles.
Locate the transfer case at the back of the transmission. Locate the lower drain plug and align the drain bucket beneath it. Remove the plug with the appropriate tool. Allow the fluid to drain thoroughly. Clean the plug with carb or brake clean spray and wrap the threads with thread seal. Replace any copper washers present. Replace the plug and tighten (but not enough to squash the copper washer).
Remove the fill plug on the transfer case (usually located a few inches above the drain plug). Remove it, clean it, replace any copper washers, and wrap thread seal around the threads. Place the hand pump into the bottle of appropriate fluid and fill to the proper level (refer to the owners manual). Replace the plug and tighten (but do not squash the copper washer).
