When I blew my rear end I thought was my transfer case. I changed fluid on the spot. Quick tip, I'm super cheap... I drained into a CLEAN catch pan. Then provisioned a filter funnel with shop towels and a filter. I ran all the fluid through the filter and into clean containers. My fluid was fine so I reused it haha I was able to see if there were any sizeable metal pieces in shop towels after.
Also my catch pan is black as most are... I can shine a bright light on it to further illuminate metal pieces.
Curious did you get it figured out? Never fun to do unexpected repairs

As you noticed from other thread, my daily driver is acting up... and sitting in the corner is my completed / built 4L60E ready to be installed in truck. I'd MUCH rather be installing the transmission than fixing coolant leak