If it's only a slight noise every once in awhile or at a certain speed and goes away, then that's not a huge deal. But again, I understand your car is not crazy modded but that tcase itself is. It's like you putting a FP turbo on your car and they recommend VR1 20w-50 oil and you question the recommendation like, "but my motor isn't built so why such extreme oil?" Protect the parts/mods with what it needs. If it isn't stock, then stock fluids might not be the best option for it. Just my OP.

 

Have you checked the slop in the rear diff? I think you will find it is out of spec. Fix it or listen to the noise, I don't think it matters.