first make sure the diff isn't too full. on level ground pull the rubber fill plug. if oil comes out it was too full. it is way too common for people to overfill them, and it will cause problems like this, and also leaky axle seals.
i'd bet its just too full. let all the fluid come out the fill hole, then reinstall the plug. clean it off w/ brake clean or degreaser, and see if it happens again.