Rear Differential Fluid Service
Drain and refill the rear diff after water crossings or every 30k miles. Includes the fill-plug-first rule that saves you from a very bad afternoon.
Clean the diff cover first
Spray the housing and both plugs with brake cleaner and wipe it down. You do not want grit falling into the fill hole when you pull the plug.
Loosen the fill plug before draining
Crack the upper fill plug loose first. If it's seized and you've already drained the diff, you now own a truck you can't refill. This rule applies to every gearbox on the planet.
Drain and inspect
Put the pan under the drain plug and remove it. Watch the oil as it comes out — milky means water got in, and metal glitter means you should be looking at bearings.
Refill to the fill hole
Fit new crush washers and refit the drain plug to 36 ft-lb. Pump oil in through the fill hole until it just starts to weep out, which is the correct level. Refit the fill plug to 36 ft-lb.
Verify and note it
Take a short drive, then check the housing for weeping at both plugs. Write the mileage and date on the housing with a paint pen — future you will appreciate it.