I changed the oil in my '07 F150's diffs. I put in Amsoil Severe Gear 75W140 in the rear as specified. As per Amsoil recommendations I omitted the additive, but my diff "vibrates" when accelerating from a stop. I have since added a bottle of Amsoil Slip-Lok, but the problem persists. It is much better, but still there. Should I add a second bottle?