If this was brought up, and I missed it, I apologize.
My son has "borrowed" my Ranger for quite some time. Not too bad, he is replacing worn parts on the front suspension as I get him the parts. He also has changed the fluids in the diff., transfer case & transmission. When I asked him if he needed the additive for a limited slip differential, his reply was "It's Possi all the time."
OK needless to say I have not heard of this. This is a '98 XLT 4WD.
Question:
Was such an rear end put on Rangers in '98, and would it not still need the additive?
I'd ask Chris, but he's busy doing Chris stuff

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Black '98 XLT 4.0 liter, super/extended cab, 4X4, 5 speed manual

*Mods & Pseudo Mods*
Sony head unit, Expo OHC, Uniden CB, 16" Wheels from a 2002 Expo Sport Trac/255-0R16 tires

unless somebody wants to discuss this, I got my answer, so the moderators are free to delete this.

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Black '98 XLT 4.0 liter, super/extended cab, 4X4, 5 speed manual

*Mods & Pseudo Mods*
Sony head unit, Expo OHC, Uniden CB, 16" Wheels from a 2002 Expo Sport Trac/255-0R16 tires