The FEH is classified as a
The FEH is classified as a flat tow vehicle with all wheels down up to 75mph. So yes, "N" coasting is okay with the engine Off or running. I've been doing it for 5 years in my '05 FEH and for 28,000 miles in my '09 FEH with no problems.
The engine in the '11 FEH will stay running or start above 44mph. For fuel economy, neutral coasting when the engine is running is worst than "D" coasting. You would have saved more gas going down that huge mountain with the engine running above 44mph or in EV below 44mph - 40mph in "D" or in "N" below 40mph in EV mode.
If you had stayed in "D" going down that mountain above 40mph you would have been in aggressive fuel-cut or what Ford calls "Deceleration Fuel Shut OFF" (DFSO). Fuel is cut by the PCM (computer) in DFSO and the transmission is using the wheels to keep the RPM's at an idle using no fuel. If you press on the gas pedal during DFSO, the PCM will smoothly take control and start delivery of fuel with the fuel injectors for a smooth acceleration again.
If you want to learn more about hypermiling and FEH techniques, join cleanmpg.com and read our articles. I suggest if your serious about increasing MPG, order a Scangauge II with a member discount there. You also can start a vehicle Mileage Log for every vehicle you or your wife drives. My Wife's 2011 Explorer now has a Lifetime MPG of 22.6mpg with 2,400 miles and my '09 FEH is at 53.3mpg with 28,000 miles.
You can learn other techniques about your pick-up also because the site is for every vehicle made.