Last week I was driving in soft deep sand on the beach. I had it in sand and disengaged the DSC and as I was going along and started to bog down a bit there would be a clunk from the rear and it seemed like something was slipping. I'd ease off the gas and it was OK and then it would happen again, just one clunk and kind of a slip. I selected deep ruts and it did the same thing. I couldn't turn around and go back for a couple of miles and it did it once or twice on the way back. When the sand firmed up, no problem. Any ideas on what was happening or how to avoid it in the future. No problems off the beach on the road or in the snow that followed.