2004 Ranger FX4 Level II, these use a torsen carrier in the rear axle. So on that note and given how a torsen works, any idea why Ford says you need to use the same friction modifier that is used on a posi carrier? I understand why it's used in a regular posi, but in a torsen there are no clutches; it's effectiveness does not dimish with miles like a regular posi. Comments?

*For readers that don't know what I'm talking about, here's a link that expalins what it is: http://torsen.com/files/Torsen T-2 Technical Sheet.pdf

Interestingly, no response after three days. Therefore, bump.

Wish I could answer your question, all I know is it does call for it. Here is Fords discription and operation.

The rear axle assembly has the following features:

An integral-type housing hypoid gear design (center of the pinion set below the centerline of the ring gear).
The hypoid ring gear and pinion consists of a ring gear and an overhung drive pinion which is supported by 2 opposed tapered roller bearings.
Pinion bearing preload is maintained by a collapsible spacer on the differential pinion shaft and adjusted by the pinion nut.
The rear axle housing consists of a cast center section with 2 steel tube assemblies and a stamped differential housing cover.
The differential housing cover uses silicone sealant as a gasket.
The differential thrust plate is retained by a threaded differential thrust plate lock bolt assembled to the differential case.
The differential case is mounted in the rear axle housing between 2 opposing differential bearings that are retained in the rear axle housing by removable bearing caps.
Motorcraft SAE 75W-140 Synthetic Rear Axle Lubricant XY-75W140-QL or equivalent meeting Ford specification WSL-M2C192-A is used in this assembly.
Differential bearing preload and ring gear backlash are adjusted by differential bearing shims located between the differential bearing cups and the rear axle housing.
The TORSEN® limited slip differential is not to be disassembled and a new differential must be installed if worn or damaged.
The rear drive axle receives rotational torque from the driveshaft. The direction of the rotation determines movement of the vehicle. The rotation enters the rear drive axle through the drive pinion flange, turning the drive pinion and rotating the ring gear. The ring gear is bolted to the differential case. The TORSEN® differential case houses the parallel axis planetary helical side gears to which the axle shafts are splined. It is the unique arrangement of the differential that allows the continuous, uninterrupted torque output and allows the axle shafts to be driven at different speed when needed, such as when driving around corners. There are no clutch plates or discs in this axle design. The TORSEN® differential requires a friction modifier that must be installed with the axle lubricant. The axle shafts are retained in this differential with the U-clips, which can be removed when the thrust plate is removed.

that style diff is the same style as used in alot of front wheel drive trannies. if i remember correctly, there is metal on metal friction surfaces and that is why the fluid is needed. the friction surfaces are basically where the back of the spider gears and carrier housing would meet in a traditional setup.

Thanks for the heads up guys.