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).
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