Just completed this in about an hour.
First off, thanks so much for the great photos and write-up on this one. My dealership wanted over $300 to do this simple maintenance, which amazes me as it was every bit as easy as an oil change.
I opted to just use the drain plug as shown in the photos as would have no idea what warning signs to look for anyhow if I were to be looking directly at the gears. Using a socket wrench with the stubby 3 inch extension worked perfect for extracting/installing the drain plugs on my Dana 44s. The fluid in the rear differential looked a bit like chocolate milk when it came pouring out, and the fluid in the front looked more like a dark amber honey. I replaced the fluid in both with 75w90 Mobil 1 Synthetic.
Filling the rear differential was simple. As for getting the fluid into the front differential, in my garage I found some 1/2 inch clear flexible hose that I had bought at Lowes and cut off a 8-10 inch piece which fit snugly over the Mobil 1 spout.
The my differentials with the factory covers seemed to only take between 1.33 - 1.5 quarts until they were full up to the fill holes.
Overall an easy project that cost me about $40 when all was said and done.