Vehicle's rear end has a knocking/clicking sound whenever drive slow or drive in reverse

Hi! Mainly hear and feel it in reverse. Sometimes it's to the point that I can feel it resisting. I continuously check the rear differential fluid, but most of the time it full. When it does need fluid, it's just a couple table spoons if that? Any ideas on what it could be? It's been a problem for about a year now. Its slightly getting worse but at a slow rate. I hate the idea of having to replace the entire rear end. Thank you so much for reading!