I have a question, I'm about to switch all fluids to Amsoil in my 97 F150 4x4. The problem is, I know from the manual what the front diff. needs but what about the rear and how do you tell if an additive is needed for trac-lock or if you even have one. It gives a Ford number in the manual but not a weight. I just want to order the right stuff. Any help would be appreciated