If you have a conventional axle in your 1997 Ford Explorer, you'll need 80W90 axle lubricant. Limited-slip and Traction Loc differentials require 4 ounces of additive friction modifier. Explorers with the 4.0-liter engine and 3.73- or 4.10-to-1 rear axle ratios, as well as those with the 5.0-liter engine, require the synthetic rear lubricant. Those that use the synthetic lubricant are "lubricated for life" according to the folks at Ford. The lubricant shouldn't need changing unless the axle was submerged or the axle or differential require repairs.
Park the Explorer on solid, level ground. If it's equipped with air suspension, locate the air suspension switch in the rear jack storage area and turn it to the "Off" position.
Chock the rear wheels. Lift the front of the vehicle -- one wheel at a time -- with a jack under the jacking brackets located on the frame behind the front wheels. Support the truck on jack stands.
Lift the rear of the vehicle -- one wheel at a time -- with a jack under the jacking points between the U-bolts on the rear axle, and support it on jack stands. Wipe the rear differential assembly clean, using a clean shop rag to remove accumulated dirt or debris.
Position a drain pan beneath the rear differential. Remove the rear differential fill plug, using a ratchet and extension. Loosen, but do not remove, the differential cover bolts, using a ratchet and socket. Pull the bottom of the cover away from the housing and allow the fluid to drain. Remove the cover bolts and cover.
Wipe any accumulated metal salt out of the bottom of the housing, using a clean shop rag. Cover the gears inside the housing with a clean rag. Clean all oil and gasket sealant from the cover and housing mating surfaces, and wipe out the inside of the cover.
Apply an unbroken bead of silicon rubber to the housing cover mating surface, between the boltholes and the inner edge. Within 15 minutes of applying the bead, install the cover and bolts, then torque the bolts to 28 to 38 foot-pounds, using a foot-pound torque wrench and socket. Wait an hour before you continue.
Fill the differential until the fluid level is one-quarter to one-half inch below the bottom of the fill hole. Install the filler plug and torque it to 15 to 30 foot-pounds, using a foot-pound torque wrench and extension.
Lower the vehicle to the ground. Turn the air suspension switch to the "On" position. Test drive the Explorer and check the rear differential for leaks.
