Differential lube.

Ford C8AZ-19B546-A stuff is excellent. Even Chevy guys use it.

Is the rear differential chattering around corners or dragging a tire in tight corners? If not, why bother? Some people purposely don't use the friction modifier so that the clutches will grip better. Without friction modifier the trac-loc will act as though it is shimmed tighter - it will be less likely to spin only one wheel.

If you didn't know what was in the rear diff I would recommend changing all the fluid. It should be changed every so often as part of regular maintenance. You can buy fluid with the friction modifier additive already in it. Then you don't have to worry about mixing. However, it's possible the previous owner put lube without friction modifier in because the trac-loc clutches were getting worn and slipping. Without the friction modifier the trac-loc clutches will grab a little better.

If you really want to add the friction modifier, put the entire bottle in. That is what you are supposed to do. Don't be surprised if your trac-loc doesn't function quite as well (you might end up with one wheel peel).

Edster said:

4oz is what the factory manual says. So I added it. Iff i get one wheel peel, I guess I'll change out the lube and not add the Trans X.

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It's easier and takes the guesswork out, if you just use the oil with the friction modifier in it. I have never had a case where I couldn't find it.

Do not use synthetic oil with a torque sensing differential.
Use gear lube in it and be sure to add 4 oz of friction modifier to the lube