Assuming you have a factory tow package (sounds like you do), you should be good. Your tow rating is 1000 pounds higher than my Explorer. Your rear end ratio is the same. Your engine is bigger, which should mean more torque (I haven't checked the numbers).
I have pointed out that my Explorer works quite hard, and slows down quite a bit, as we approach my personal benchmark, the west entrance to the Eisenhower tunnel. (I hold the RPM to 4000). I would expect that you would do better, but your Navigator will still be huffing and puffing pretty seriously. The problem isnt the grade - it is the altitude. A naturally-aspirated vehicle just can't breathe up there (neither can I!) Your 4V engine design should help in this area. I would love to have a supercharger to restore sea-level air pressure, but years of searching have yielded nothing.
Enjoy! You should be good to go.
Bill
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