Just returned from vacation...put 924 miles on the truck, no sign of the dreaded "Service 4WD" message. The truck performed well and ate up miles beautifully, very comfortable on the highway. I ran it in 2WD all the time.
Now that I am home and close to service I will run it in Auto to see if that triggers the issue again. If not, I can only assume that replacement of the transfer case position sensor did the trick.
Side note: The truck was loaded with 6 people, lots of gear, and towing a waverunner - averaged 18.0 mpg for the trip.