Live rear axle in an Expedition/Navigator would mean a new unique rear floorpan with much high load floor and compromised rear seating package. Not going to happen!
The new Expedition/Navigator are likely to be 2018 models with aluminum structure similar to F150. It will be interesting to see the transition plan for KTP regarding SD and Expedition /Navigator as I don't believe that Aluminum and steel vehicles can go through the body assembly equipment at the same time.