Everything sounds perfectly normal. Since the rear axle does 95% of the driving, it will have a lot more wear on it then the 30 will. Also, because the 30 is freewheeling on one side nearly its whole life, the spider gears will have some amount of wear at 204K, but unless they are chipped or have a lot of play, they are more than likely fine. Dirty diff fluid always has the looks like almost burnt ATF, or metallic motor oil. This is normal; there is lots and lots of iron to wear and not a problem. As long as the fluid is to the check hole, then the gears are getting lubed and you have nothing to worry about.

I change my diff fluid whenever I go through water that goes 3/4 as high as the tires, and the t-case and tranny fluid whenever I go in water as high as the doors. Diff/tranny/atf fluid is so cheap that even if I don't go mudding or through creek crossings, I do it every year at the end of the rainy season.