My AWD front diff has the same problems. Somewhere along the way the diff vent tube fell off the plastic fender well. Dangling upside down, breathing water, salt, sand for what looks like a few years. Here is what I found during winter when I was looking for a mysterious leak on the garage floor. It was the pinion seal puking fluid, which lead me to this..
The vehicle was serviced by the dealer during this time. They never noticed greasing, changing the oil, changing all fluids. Too late now the damage is done. The drain bolt has the same metal as yours, fluid looked like this...
After 5 Mobil 1 75w90 drain and fills over 2 years, its alive. Not leaking. The drained oil is clearer, but still get the metal buildup on the bolt like yours, still get silver in the gear oil. Something is toast, the end is near. I do a lot of DIY, but rebuilding diffs require skills I don't have. Setting the preload, backlash, etc. are not something I want to learn on this truck. I've been researching, there are a few that rebuild my 3.73 front diff on eBay and sell for +-$1,000. From what you mentioned in your post, you seem capable to shotgun the diff and be done with it. I do think the front diffs in our GM AWDs are a weak spot, and should see yearly maintenance. I just changed the rear diff fluid which is also 75w90 syn, and at 155k the fluid was "good". No silver fluid, hardly anything on the magnet.