Tools & materials (brief): jack stands, wheel chocks, drain pan, ratchet/sockets, torque wrench, pry bar, gasket scraper, brake cleaner, rags, RTV or new gasket, new differential fluid (manufacturer spec — typically 75W‑90 GL‑5 for TJ), friction modifier if limited‑slip, replacement seals as needed. Service manual for specific torque/specs.

Ordered procedure with theory and how each action fixes faults

1) Safety & ID
- Action: Secure vehicle on level ground, chock wheels, lift and support on stands. Identify front/rear type (Dana model) and whether limited‑slip is present.
- Theory: Safe support prevents collapse. Knowing which differential and whether limited‑slip matters for correct fluid type and repair approach.

2) Preliminary checks & symptom triage
- Action: Note noises (whine at speed, clunk when shifting, leaks), check fluid level at fill hole if present.
- Theory: Symptoms point to likely faults: whining ≈ gear misalignment or worn bearings; clunk ≈ excessive backlash, worn splines/axle or ring gear teeth; leaks ≈ bad pinion or axle seals; metal in fluid ≈ bearing or gear wear. Correct diagnosis narrows the repair needed.

3) Drain fluid
- Action: Remove filler (if present) then drain plug or remove cover to drain. Catch fluid and inspect color/odors.
- Theory: Removing old fluid exposes contaminants. Fluid carries metal, abrasive particles and degraded additives; draining eliminates contaminants that accelerate wear and lets you examine debris to judge wear severity.

4) Inspect drained fluid & magnets
- Action: Check fluid for metal flakes, copper (bearing wear), or large chunks. Clean and inspect magnets in housing/cover.
- Theory: Small fine metallic sheen is normal; lots of copper or chunks → catastrophic bearing/gear failure. Magnets collect ferrous debris; cleaning them removes abrasive particles that would otherwise continue to abrade bearings/gears.

5) Remove cover (if deeper inspection needed)
- Action: Unbolt cover (or remove axle) to expose ring gear, pinion, carrier and bearings. Clean housing.
- Theory: Visual inspection reveals patterns of wear, pitting, scoring, heat discoloration, broken teeth, and bearing condition. You can’t reliably decide between a fluid/seal service and an internal rebuild without this inspection.

6) Visual and tactile inspection of gears & bearings
- Action: Rotate driveshaft and observe ring & pinion mesh, check backlash, rock carrier to assess play, spin pinion to feel bearings.
- Theory: Proper gear mesh is a controlled contact patch on ring gear teeth; bearings should roll smoothly. Excessive play/backlash or rough bearings indicate worn components. Gear tooth patterns (location of contact) indicate pinion depth and backlash settings — wrong pattern → noise and accelerated wear.

7) Decide repair scope
- Action: If gears & bearings look healthy (no heavy wear, no chunks), replace seals/gasket and fluid only. If bearings are rough, pattern is incorrect, or significant metal is present, plan a rebuild (replace bearings, shims/crush sleeve, ring & pinion as needed).
- Theory: Fluid and seal replacement fixes contamination and leaks, which often resolves minor problems. Noisy or damaged internals require setting pinion preload and backlash and replacing worn parts; simply adding fluid won’t correct misalignment or bearing play.

8) Replace seals & gasket / reinstall cover
- Action: Replace pinion seal and axle seals as needed. Clean mating surfaces, install new gasket or use appropriate RTV, torque cover bolts to spec.
- Theory: Seals keep lubricant in and contaminants out. Leaks let lubricant escape and allow dirt/water in, causing bearing and gear failure. Proper sealing restores and preserves lubrication, preventing recurrence.

9) Refill with correct fluid (and modifier if required)
- Action: Refill to correct level via fill hole with manufacturer‑specified gear oil. Add friction modifier if differential is limited‑slip and the OEM requires it.
- Theory: Gear oil provides film strength, reduces friction and cools parts. GL‑5 oils have extreme pressure additives for hypoid gears; limited‑slip units require friction modifiers to allow clutch packs to behave properly. Correct fluid restores lubrication and proper clutch operation, eliminating chatter and wear from wrong oil.

10) Torque, fill, and leak check
- Action: Torque drain/fill/final bolts to spec, wipe clean, run engine/rotate driveline, check for leaks.
- Theory: Proper torque ensures fasteners withstand loads without loosening and that seals compress correctly. Running the driveline lets fluid circulate and reveals leaks and noises.

11) Road test and re‑inspect
- Action: Test drive under conditions that produced symptoms, then recheck fluid level and for leaks.
- Theory: Confirms symptom elimination. Some faults only show under load and may require further adjustment or rebuild.

12) If noises or excessive metal persist — rebuild theory & what to change
- Action (theory level): Rebuild involves replacing worn bearings, setting pinion bearing preload (shims or crush sleeve), installing a correct depth ring and pinion, and setting backlash via carrier shims or adjusters. Use gear marking compound to verify contact pattern and adjust until the contact patch is centered where manufacturer specifies.
- Theory: Ring & pinion geometry must be precise. Pinion depth controls where the pinion tooth contacts the ring tooth face; backlash controls lateral clearance between teeth. Incorrect pinion depth/backlash causes edge loading, whine, heat and rapid wear. Preload on pinion bearings ensures they stay in position under load — too little preload → movement and noise; too much → bearing overload and premature failure. Replacing worn bearings and reestablishing preload and backlash restores correct gear tooth engagement and bearing life, permanently fixing noises caused by misalignment or wear.

When a simple service fixes the fault vs when rebuild required
- Fluid/seal change likely cures: leaks, contamination, minor noise or slight chatter if due to old fluid or missing friction modifier, and prevents future wear if done regularly.
- Rebuild required when: heavy metallic debris, rough bearings, broken/battered gear teeth, pronounced whining under load, or excessive backlash. Replacing fluid/seals only is a temporary measure in those cases; only correcting geometry and worn parts fixes the root cause.

Quick troubleshooting cues (what the observed fault means)
- Whine at speed that changes little with throttle: typically ring & pinion gear misalignment or worn gears.
- Whine/noise changes with throttle and with load: pinion bearing or pinion preload issue.
- Clunk on engagement or shift: excessive backlash, worn carrier shims, loose axle splines, or worn U‑joints.
- Visible leak at pinion yoke: bad pinion seal/crush sleeve.
- Metal flakes/copper in oil: bearing wear — expect rebuild.

Final notes
- Use the factory service manual for torque, shim thicknesses, preload specs and gear pattern targets — the theory explains what to do, but the exact adjustments are spec‑sensitive.
- Dispose of used gear oil properly. Follow safe work practices around lifted vehicles and heavy components.

That’s the procedure in order with the mechanical theory and how each repair action corrects the underlying fault.
rteeqp73

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