Definitely. I don't know the exact honest amount but you need to keep in mind that there's no locking hubs or axle disconnect on the Rangers so you're turning the front axle and driveshaft all the time. The only place its not locked in is inside the transfer case, until you engage it into 4wd. 2wd doesn't have the extra couple hundred pounds of front axle, CV shafts, nor transfer case so thats a fuel savings of ~1 mpg right there. No comparison between the two.
You can laugh at the guy who gives you grief for buying a 4wd when he's stuck in sand or snow, simple click of the switch and you drive away while he waits for help. 2wd is lame. LR
'09 Ford Ranger FX4 with a mean owner