That gas mileage does sound very poor. My math isn't great but I'm guessing that you are achieving roughly 10 US mpg (perhaps a little more). The 4.0 in your 08 is different from the one in your 1992, your 08 has an OHC. One factor could possibly be weather or not your engine is broken in yet. How many miles does it have on it? Is your transmission manual or automatic? I found out after surfing the forums that I've been shifting too low and when I changed that habit I gained about 1.5mpg. I have an 03 (xlt) and checked the axle code. It came stock with 410s.
jdanger- Be proud of 20+. The most I have ever got is 20.2mpg on a long highway trip going between 70-75. How fast (slow) were you going?
I've been averaging 14-15 in mixed driving more often 15 now that its warmer. Are you letting your engine warm up on the first start? How much are you using 4wd (we had a blizzard here 2 weeks ago so I had to use it)? Worst I have gotten was 12mpg and that was letting my truck warm up every morning, using 4wd and shifting too low.