I have an 2011, for mpg's I've seen as good as 19mpg fully loaded with two dirt bikes from Durango CO to Moab UT and in the low 20's from Durango to Denver. It gets about 15-17ish in town. I wouldn't worry too much about the front diff unless you are unable to drive with some finesse. Even still, you can break a chrmo D60 if drive it like you stole it and don't control wheel spin.
I bought my 2005 CC NISMO 4x4 about a month ago so I can't speak to long term reliability. I bought it with a howling timing chain so that was the first thing I took care of. I also swapped the radiator so I didn't have to have the nagging transmission worry in the back of my head every time I drove it. I negotiated both of those repairs into the price and as long as the truck goes for a while I'm really happy with the deal I made. mike r is 100% right. If you're looking for info on a Frontier, go to clubfrontier. The problems with the earlier models are well documented and there's a ton of information there.
My only real complaint so far is that the gas mileage isn't great. I get around 15 mpg but most of my mileage is in the mountains. I'm looking forward to taking the truck on a road trip so I can see where it all lands.