$476 to assemble an 8 3/4 diff

Seems more than reasonable to me for something you obviously cannot do yourself or you wouldn't be taking it to a shop. That wasn't an insult, just a realistic statement about what's happening. I'll echo what others have said. If you think it's too high, learn how to do it yourself. It takes guys years of experience to learn how to set rear axles up properly. Call Moser, Strange, or any other big outfit and see what they charge to assemble one.