GMC trucks are hard-working, reliable vehicles that can take as much punishment as their owners dish out at them. Part of the drive line on these trucks that often goes through the roughest and most intense punishment is the differential. The differential is the part of the transmission drive that contains the gears that spin the axle shafts. While the workings of the differential are fairly intricate, there are a few maintenance routines that, if performed regularly, will assure a long and useful life for your GMC. One such job is replacing the differential fluid. Changing the differential fluid is not difficult and can be performed in a single afternoon.
Park the truck somewhere level and jack the front of the vehicle up.
Crawl under the rear of the truck and locate the differential, which looks like a round case connected to the truck's driveshaft. On the bottom-center of the transfer case is the drain plug; remove it with a socket wrench.
Place the waste pan under the drain hole and allow the fluid to drain into it. Replace the drain plug and remove the bolt for the fill hole on the right side of the differential case using a socket wrench.
Insert the plastic tube from the fluid pump into the hole and pump the fluid into the transfer case until it is full. Replace the filler hole bolt and lower the truck off of the jack.
