Removing my front driveshaft to fix the "angry sparrows" problem on my TJ bought me a little more than 1 mpg. Obviously this makes your rig a 2WD, but if you're using it as a daily driver, you'll get some coin back if you do this between off-road excursions. Switching to straight 87 also did about 1.1 mpg better. We've got cheap 87 (with 10% ethanol) in Iowa and straight 87 (no ethanol). Last tank I ran 17.7 mpg (mixed city and highway, commuting) with my 2006 Rubicon. First time I've beat the EPA highway rating!
Having locking hubs would eliminate the need to remove the driveshaft for savings, but the last time I priced those out for my front D44 they were in about the $1500-$2000 territory
"I got a shotgun, rifle, and a 4-wheel drive and a country boy can survive"
- Hank Williams Jr.