I clean them out using compressed air. I wrap the diff in a towel and put my air compressor nozzle through it. Then I hold the towel wraped around the diff and air nozzle and blast it with air. All the old diff fluid is on the towel and the diff is generally cleaned and ready for refilling. Its fast easy and effective but you gotta have a compressor.
I dont like having a bunch of chemicals around because I have 2 kids. I always worry a little about the type of effect they will have on the plastics too.