Re: 1992 Bronco Rear End Chatter While Turning
It sounds like you are on the right track. This is most likely a problem with the limited slip clutches.
Now I have heard some techs say that they add two bottles of friction modifier just to be safe and avoid return complaints of "chattering rear end".
But what does the friction modifier do? Really, it enables the clutches to work at all, not to disengage. I would argue that with no friction mod that you would essentially have an open axle. It sounds to me like your clutches won't disengage when hot, causing a lockup of the diff.
I think that your problem is not with the fluids (as it sounds like you have done everything right). My thought is that you either have the tolerances too close in the clutch pack (a difficult thing to change), your clutches are worn (time to replace), or they are simply dirty.
As the latter is something that is cheap and easy to try to fix, I would recommend draining the fluid again, and use several cans of brake-clean to to get every little bit of grime out of the clutch packs as possible. the little bits of gunk could be interfering with the clutch operation and causing lock up at higher temps.
I might also suggest that you consider synthetic diff fluid. ford has moved to using 75w-140 instead of the 80w-90 and friction mod. The Synthetic fluid replaces any need for friction mod from what I understand. It is the OEM on any new limited slip axles.