'08 Expedition limp mode?

[cpl624]

My wife has an 2008 Expedition EL 4x4 limited with about 45K. About a month ago she was driving to church during a snow event and had the 4x4 in high. She said for no reason or warning the truck went into limp mode. She was close to church so drove the rest of the way in that mode. We she came out to leave the truck drove fine, with no apparant change. Tonight on her way home (In a snow storm) she was driving in 4x4 high again and the same thing happened. She pulled into a parking lot and called me and the truck died. She called me and I arrived 10 minutes later. Put her key in the ignition and it fires right up and drives normal. Both times it happened it was snowing and in 4x4 high. Could be a coincidence???? This truck has no mods, has been maintained by the book, and never driven hard. Any suggestions?