do I need an additive in my 8 3/4 sure grip?

Can somebody tell me where I can get this additive? :banghead:

My 98 Dodge Dually is making noise in the rear. Would like to drain current fluid, add some of the Lucas Rear Oil and add the additive.

Rear gets noisy towing in 90+ degree heat in Summer. Has knocking sound when I pull into refuel. I think it is on the DOWNWARD SPIRAL.

I hear the Ford additive is the best, containing whale oil.