If your vehicle is showing thick, dark gear oil leaking from the center of the rear axle, here are the most likely causes and the parts you'll need to fix each one.
The seal where the driveshaft enters the front of the differential wears out over time, allowing gear oil to seep past the spinning shaft.
The rear cover gasket or RTV sealant has degraded. This is often caused by improper cleaning or insufficient sealant application during a previous fluid change.
A blocked vent tube causes pressure to build up inside the axle housing as it heats up, forcing fluid out through the pinion or cover seals.
Worn internal bearings allow the pinion shaft to vibrate and move excessively. This movement repeatedly destroys the pinion seal, causing persistent leaks.
A severe physical impact from off-roading or a catastrophic internal gear failure has cracked the metal housing, allowing fluid to drain out.