At 26K seems a bit early for rear differential fluid..especially since I believe the top 3 all come with synthetic gear lube in rear diff..
However.. I changed mine on my 2007 diesel duramax at 50K first 5ime and since then do it every 30K...
My initial rear diff gear oil was very dark and it’s what I expected to see.. no signs of any metal filings on drain plug... I have changed rear diff gear look two more times since my initial at 50K. I currently have 121K on the truck
The diff gear oil color was not near as dark as factory gear oil was and I’ve never seen any metal flings, etc on drain plug during changes.
26K for brake fluid change is just someone want8ng to take your money...no reason what so ever.
And down the road you can partially change out that brake fluid b6 removing reservoir cover and use a turkey Blaster and siphon out the old brake fluid from front and rear reservoir wells being careful not to draw out enough to expose fluid hole in bottom of each reservoir well..
Then simply fill back up with appropriate disk brake fluid and be down with it