The chatter is caused by the cone clutches grabbing as the rear end differentiates axle speed in a low-speed turn. Regardless of where you get the posi additive, Ford, Chrysler, or GM, it's all the same stuff. Mix it according to the directions on the bottle, and it should cure your problem. At heavy throttle the axles will lock if any slippage is present, so the additive is just to prevent chatter at low speeds with little if any power applied.
Jerry

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