The Dana 44 TTB front takes about 3.5–4.0 pints of 80W-90 GL-5 gear oil. The Ford 9-inch rear takes about 5.5 pints (a touch over 2.5 quarts). Add Motorcraft XL-3 friction modifier if the rear has a Trac-Lok limited slip — otherwise the clutches will chatter on tight turns.
The Dana 44 TTB and Ford 9-inch are tough axles, but they aren't immortal. Trail use, deep water crossings, and 35,000-mile service intervals all push moisture and metal into the diff fluid. If your truck came out of a creek, change the diff fluid before storing it overnight — water in the fluid emulsifies to a milky sludge and corrodes the bearings while the truck sits.
The front axle (Dana 44 TTB) has a fill plug only — no drain plug from the factory. To change it, you suck the old fluid out through the fill hole with a fluid pump, then refill. About 3.5–4.0 pints is the target. If the old fluid is gray, milky, or smells burned, you'll want to pull the differential cover (if your axle has one — TTBs don't all) or back off the carrier and inspect.
The rear axle (Ford 9-inch) is the bigger procedure. The 9-inch is a third-member design — meaning the gear set unbolts as a unit from the front of the housing. For a fluid change, you don't pull the third member; you remove the rear cover instead. That gives you a chance to inspect ring-and-pinion contact pattern, scrape the magnet, and re-seal. Some 9-inch housings don't have a removable cover (the integral-housing earlier trucks) — those require pulling the third member to drain. Check what you have before you start.
For Trac-Lok limited slips, the friction modifier is not optional. Skip it and the clutches will chatter — you'll hear a banging or popping noise from the rear axle in low-speed turns. The modifier is a 4-ounce bottle that goes in with the fresh gear oil.
Synthetic 75W-140 is worth the upgrade if you tow, run 35"+ tires, or do any sustained low-range work. The extra film strength is real, and 75W-140 holds up to heat soak better than conventional 80W-90 in slow trail miles. For a daily-driver Bronco in Phoenix that sees pavement and dirt roads, conventional 80W-90 is fine.
| Part | Vendor | Est. price |
|---|---|---|
| Gear oil — 80W-90 GL-5 (1 quart x 5 needed) | Valvoline / Royal Purple / Lucas | ~$35 |
| Synthetic 75W-140 gear oil — for trail/towing rigs | Royal Purple Max Gear | ~$55 |
| Friction modifier (Ford C8AZ-19B546-A for Trac-Lok) | Motorcraft | ~$8 |
| Diff cover RTV (anaerobic gasket maker) | Permatex Ultra Black | ~$8 |
| Rear diff cover with drain plug (optional upgrade) | Mag-Hytec / Ruffstuff | ~$95 |
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.