tech says the front diff turns with or without the 4x4 engaged. After all this speculation, noise, or 2 noises were from the front diff bearings having worn, and a slightly crooked driveshaft.

4 days in the shop, 3 mechanics test driving and listening, 4 days driving a loaner Focus (real fun) and $0.00 warranty bill. Now lets drive the bast--d to Florida!
Thanks for the comments and enjoy what's left of winter. Lookin forward to washing the salt off and seeing the paint again.

 

Hey, you got a loaner. I didn't when I had warranty work done on my '15 w/8K miles on it. Had to pay for a rental for 2 days......

 

The front differential bearings wore out after 18,000 miles? Seems fishy.

 

It sounded like I was dragging a grocery basket......

The auto hubs are vacuum engaged. There is a vacuum relay by firewall drivers side. Early models would get water in from hood. Aftermarket is a relay with a metal shield. $14.

 

My wife's 15' F150, 46,000 mi, loaded truck, auto 4x4, has a similar noise coming from the front end. Dealer service rep told us that numerous F150's with the auto 4x4 have been coming in with same noise and Ford has no fix for it.

 

Dont take any bs. First time, they told me it was because I had rustproofed the undercarriage and there was dirt stuck to hardened rustproofing...make them start changing parts till they find it or go to another dealer!

 

I can't say that there is no rotation of the front drive shaft when in 4x2 mode when driving but I can say that the front shaft is "not" being powered from the transfer case in 4x2. Just crawl under the truck with the engine running and in 4x2 and you can rotate the shaft freely by hand. For that reason I doubt a bad bearing in the front differential is a source of noise while in 4x2 mode.