It will be perfectly fine.
80/90 is more for general use.
75/145 is for more "heavy duty" (or high torque) applications, meaning that you tow a lot or that the gears are under that much more stress. It handles the heat a bit better and will break down at a much hotter temperature. It will not hurt a lower torque or general use application.
For all intents and purposes under general use, unless you live where there are wild temperatures (+ or -), and especially once there are some miles on the truck and the teeth wear on eachother and the tolerances open up a bit, just about any good clean "gear oil" of relatively close viscosity will suffice. The hypoid gear design hasn't changed much since the 1930's. It's easy to over think this stuff, I don't blame you. There's an almost overwhelming amount of (mis)information out there.
*edit: I just re-read that you have the additive already, but I'll leave the next bit in just in case.
HOWEVER, you do have the L/S version, which means you may need friction modifier. If it's full synthetic, you usually won't need it. The only thing the lack of F/M will do is make the clutches grabbier, and of course, potentially wear out faster. The good news is that if you feel your L/S is unnecessarily/unsatisfactorily "grabby", you can add the modifier separately afterwards.
Again, if there's some decent miles on the truck, the clutches may be worn out as it is, and it won't matter nearly as much.