just changed the rear diff fluid with 75W 140 syntheic as req'd. then i was reading a few other posts and someone mentioned something about adding an additive to it. my truck is 4x4. do i need an additive, and what kind? what is the purpose of it?

 

I've read about the friction modifier added to the rear diff. fluid - even when using fully synthetic gear oil. When I had the rear gear oil in my '98 Expedition changed (with the recommended fully synthetic gear oil), I asked the tech about the requirement of the friction modifier (sometimes called fish oil) and he said only if it keeps making noise. The limited-slip rear would make a chattering noise in turns until I went back and had him put in the 7-8oz. bottle of additive. No more troubles.
I believe I read about it in a Hayes or Chilton's repair manual.

 

sweet thanks for the input