How do you synchronize the gears in the differential? Or do I have a different issue to address?

Thanks guy. I did find the rubber plug for filling the diff. It was buried under a 1/2 inch of built up road dirt, etc. And up close it wasn't visible. From a distance you could just barely see the outline of the plug.
New question: I drained, cleaned, and refilled with new 80/90 gear lube. I observed all the gear teeth and their movement when it was empty. All looked clean and with no noticeable irregular wear. Once refilled I still have a growling noise coming from the diff while allowing the rear wheel to spin when the vehicle is up on jack stands. How does one go about adjusting the position of the gears to make them mesh more perfectly? The gears look to possibly be replacements and I'm wondering if by chance they weren't synchronized properly. Ideas?