My 08 L gets almost 20 hiway. I bought mine in Maryland so the 4x4 was used for snow driving I assume. The only time I use it is when I'm backing my trailer up over the curb. The back tires just spin trying to pull the front tires over the curb while trying to push the trailer. It works good though. Have never had a need for it otherwise and certainly wouldn't consider taking this vehicle off road.
Well, gas mileage is great compared to my '97 F250. The 5.4 in the Navigator does much better than the 7.5 in the truck! Probably 90% of the vehicles you can buy will get better mileage, so if that's your sole criteria you'd do well to look elsewhere.
I've found my Navi L can do lots of things that those other 90% can't do, so I feed it plenty of the cheap stuff and move on.
As far as off-roading, I suppose it depends on what you want to do. These are big buses, especially the L's, so navigating (ha-ha) narrow trails like a CJ or Wrangler ain't going to happen.
The Navi's don't have low range either. But first gear in the 6-speed is plenty deep, so I don't think it's an issue. Also, it's not like you'll be slipping the clutch to get it going.
The traction control is brilliant as well, so I think you'll be able to go places you fit, even without a locker.
Ground clearance... if you have the power running boards it might be a hair better between the wheels than an Expy that doesn't (Expys could have power boards, but try to find one.)
So, hard-core rock climbing? I don't think so. Getting to the cabin, or scouting through some fields? Don't see why not. But put those chrome 20's away and find something more trail approriate. Though far from identical, here's alot of F150 underneath these.
i do some ditch diving and ripping down by the rivers which are not at all maintained for work a few times a week and im in canada so... and i drive an 07 el limited. I also live out in the country so at times the "roads" get pretty bad (mud, snow ect).
its no hard core wheeler but it gets the job done. have not got stuck yet (knock on wood)