I changed the front and rear differential fluid in my truck today. The truck has 48,000 miles and the rear fluid came out a dark golden brown and it smelled of grapes and diff lube, quite the combination of smells. The magnet in the bottom of the differential housing had more goop/fuzz than I expected but nothing big enough to actually see with the naked eye. I re-siliconed the magnet in place and let it dry as well. I cleaned/scrubbed the cover in our parts washer and flushed out the housing with brake clean. GM was nice enough to use a reusable gasket.
The front was easy as GM was nice enough to put a drain and fill plug. The fluid that drained out was thick and almost gray. I am guessing they didn't put a synthetic up front.
I refilled both with Mobil 1 75w90. I figure they are easily good for another 50k. I very rarely tow and most miles are commuting or vacation.