2013 challenger rt rear dif noise & fluid

I've recently developed a rear end noise. My car is muffler delete so I can only hear the noise, off throttle and I can only hear it at 20 mph or so and less. It is a clicking noise, (in the rear end, I believe) that gets faster and slower with speed, but is faster than tire speed. It is not a heavy click (or clunk) and I'm leaning towards thinking that I have a chipped or pitted bearing. Does this make sense or is their other common problems. Also is 75/90 the correct fluid for this? Thanks in advance.

The noise is definitely more pronounced when in neutral and the rear end is free rolling. The noise can be heard through a stethoscope coming from inside the rear dif. We drained the dif. and the oil was very dark (not burnt) with a slight brass metallic sparkle to it. There was also lots of sludge on the magnet, but no metal fragments. I was hoping to avoid a full tare down, but I think that it's going to be necessary.