Pull the cover, drain, scrape the old RTV clean, lay a 1/8" bead of new RTV around the cover flange, torque cover bolts in star pattern, and refill through the fill plug until oil dribbles out — Dana 30 takes about 1 quart, Dana 35 takes 2 quarts, Dana 44 takes about 3 quarts.
Differential fluid is one of those services that's invisible until it kills the axle. The oil sees high heat, fine metal particles from normal gear wear, and water intrusion if you ford streams or wash with a pressure washer. Change interval is 30,000 miles for street driving, 15,000 miles or annually for trail use, and immediately after any water crossing where you suspect the axle vent ingested water (cloudy or milky oil = water contamination — change it now or lose the carrier bearings).
YJ axle math: the front is a **Dana 30** on every YJ, about 1 quart capacity. The rear is a **Dana 35** on 4-cylinder and most 6-cylinder YJs (~2 quarts), or an optional **Dana 44** on a small number of trucks (~3 quarts). If your rear cover has 10 bolts and a flat-bottom shape, it's a Dana 35. If it has 10 bolts but a more rounded shape and is noticeably larger, it's a Dana 44. Check the tag on the cover or the casting if you're unsure.
The Dana 30 has a square-headed fill plug at the front-passenger side of the housing — a 3/8" drive ratchet fits directly into the square hole. The Dana 35 fill plug is a rubber-and-metal stopper that you pry off with a flathead screwdriver. Most YJs don't have a drain plug — you pull the cover to drain. Plan for that: have a new gasket or a tube of RTV, a wire brush to scrape the old sealant, and brake cleaner to degrease the mating surface. Cover bolt torque is **20 ft-lbs** for Dana 30 and Dana 35, **30 ft-lbs** for Dana 44. Tighten in a star pattern in three passes — even pressure prevents cover-flange warp.
Fluid choice: 80W-90 conventional gear oil is the factory spec. 75W-90 synthetic shifts a bit more manageable in cold weather and runs slightly cooler under load — worth the extra $6 per quart on trail-driven rigs. If your rear has the Trac-Lok limited-slip clutch pack (most YJs do not — it was optional), add 4 oz of friction modifier or use a gear oil rated for limited-slip use, or the clutches will chatter on tight turns.
Fill until oil dribbles out the fill hole with the axle level — that's the correct level by design. Over-filling pushes oil past seals and onto your brake drums. Underfilling starves the ring gear and burns the pinion bearing. Both axles have a vent tube on top — pull the vent hose and verify it isn't crushed or plugged before you drive away.
| Part | Vendor | Est. price |
|---|---|---|
| Valvoline 80W-90 gear oil, 1 qt | Valvoline | ~$8 |
| Mobil 1 75W-90 synthetic gear oil, 1 qt | Mobil | ~$14 |
| Lucas limited-slip additive (Trac-Lok rears only) | Lucas | ~$8 |
| Permatex Ultra Black RTV | Permatex | ~$8 |
| Mopar Dana 35 cover gasket | Mopar | ~$12 |
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.