Rear axle takes 75W-140 synthetic — capacity is about 2.35 quarts (75 oz). If the truck has Quadra-Drive II with ELSD, add 4 oz of Mopar limited-slip additive to the rear. The front takes 75W-90 synthetic.
Diff service on the WK is a once-per-50,000-miles job, sooner if the truck does any towing or trail work. The fluid is dirt cheap insurance against ring-and-pinion wear, and the catch — the part most owners skip — is that Quadra-Drive II uses Electronic Limited Slip Differentials (ELSD) front and rear, and those axles require 4 ounces of Mopar limited-slip additive in addition to the gear oil. Skip the additive and the clutch packs chatter, the ELSD behaves erratically, and eventually you replace the unit.
The rear axle on the WK is either a Chrysler 8.25 (3.7L V6 trucks) or a Dana 44a (4.7L V8 and 5.7L Hemi Quadra-Drive II trucks). Both have a fill-plug-and-drain-plug arrangement on most years, though some require pulling the cover to drain. Verify which one you have before buying parts — pull the fill plug first to confirm.
Capacity: rear is 75–76 oz (about 2.35 quarts). Front (Dana 30/44 IFS housing) is similar — 2 quarts of 75W-90 covers it. Fluid type is non-negotiable on ELSD axles — 75W-140 synthetic API GL-5 in the rear. The front uses 75W-90 synthetic.
Park on level ground — this matters, because the fill plug is the fluid level indicator. Pull the fill plug FIRST. If you can't get it out, do not crack the drain. (You don't want to be stuck with a drained axle and a stuck fill plug.) The fill plugs are typically a 3/8" square drive (use a 3/8" ratchet directly into the plug without a socket) or a hex socket — check yours before buying tools. Drain plug is the same style. Drain into a pan, reinstall the drain plug, then refill through the fill plug with a pump bottle until fluid runs out the fill hole. Add the ELSD additive last on Quadra-Drive II rear axles.
If your axle has no drain plug, pull the cover (10 or 12 bolts), drain, scrape the old gasket or RTV clean, lay down a fresh bead of RTV (Permatex Ultra Black), torque the cover bolts to spec in a star pattern, wait the cure time on the tube before refilling.
| Part | Vendor | Est. price |
|---|---|---|
| Mopar 75W-140 synthetic gear oil (API GL-5) — 2 qt rear | Mopar | ~$35 |
| Mopar 75W-90 synthetic gear oil (front) — 2 qt | Mopar / Valvoline / Royal Purple | ~$30 |
| Mopar Limited Slip Additive (4 oz, ELSD axles only) | Mopar | ~$12 |
| Differential cover gasket or RTV (if pulling cover) | Mopar / Permatex Ultra Black | ~$8 |
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.