You are moving a 2.8 ton (5600lb) vehicle. The removal of the front shaft won't do squat for that, get a mini car that weights half of that (Fiat 500 has 2363 lbs) if you want good gas mileage.
Of course be prepared to be squished by some guy in a heavyer car that doesn't care about mileage as much as you.
If you don't belive me, drive the Explorer to 40mph and shift in N. See how long it takes to stop? There is not a real "drag" because of spinning axles (if your brakes are not stuck).
98% of the gas energy consumed in city is to accelerate the 2.8 ton hunk of steel. And you loose that at every stop sign/light.
On highway is the wind drag. At 80mph you have 8 times more wind drag than at 40mph. We are driving a brick. Get behind a tractor trailer and coast in his draft.
On engine side, personally, I use only syntethic fluids, and have added Lubro Moly in my oil.