Actually a limited slip diff or Trac-Lok needs the additive
NOTE: Trac-Lok® differentials require Limited Slip Additive in the lubricant. Trac-RiteTM / Trac-Rite ELTM differentials DO NOT require Limited Slip Additive
Try that little bottle of additive before going any further. At this point it cannot hurt since you've already spent the time and money to change out the fluid twice.