Cass, a.k.a Dr Diff has always told me to use the Ford Racing Friction modifier. Add at least one bottle, and use 2 if you still get differential chatter. My local rear end shop recommends the same claiming that even though most dinosaur and synthetic gear oils are being produced with their own friction modifier these days.
The clutches in my Dana 60 started to chatter on turns after a while and I added a second bottle. It stopped it again.
1970 Plymouth 'Cuda #'s 440-6(block in storage)currently 493" 6 pack, Shaker, 5 speed Passon, 4.10's
1968 Plymouth Barracuda Convertible 408 Magnum EFI with 4 speed automatic overdrive, 3800 stall lock-up converter and 4.30's (closest thing to an automatic 5 speed going)
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