I have never used any of the Lucas, or much of any 'high mileage' products or additives. In general, I don't see the reason or any potential benefit except to the marketing types who live off the additional sales. Oil is for lubrication. If the gears, rings, whatever, are worn no additive or special elixir can reverse that fact. The hope that there is a 'magic cure' helps sell these products.
Looking at another thread I think you have a limited slip diff. Those need the Ford friction modifier additive or they make noises when you turn corners forcing the clutches to slip. If the clutches still work, i.e., the rear wheels won't easily turn independently of each other, and it doesn't make noises I would leave it alone. If the wheels turn easily, one at a time as mine did, then the friction plates need to be replaced. I don't have a 9" but on the smaller diffs that isn't terribly difficult if you don't mind taking a differential apart. My plates were completely shot since the original owner had limited slip for driving through the snow with chains to his ski cabin at Taos for 14 years. He did it weekly and the limited slip was needed often.