Great to hear, sounds like the differential fluid was wearing down and lacking the proper levels of friction modifiers. Good that you changed it out.
Is your 93' Lightening a manual or automatic. If I recall, isn't that a 5.8L engine?
93 Lighting is an E40D or something like that, predecessor to the 4R100. Takes old school Dexron/Mercon not Mercon V. Mercon V is known to wipe them out. What would be the Amsoil equivalent to Dexron/Mercon?