Re:dropping fuel mileage - help!
Jon, base stock gear oil gets awfully "sticky" when it's cold, and can really cause a drag on your drive train. You might consider switching to a synthetic in your differentials. I've been getting slightly better mileage than you at around 17-17.5 (calculated) doing 95% interstate driving. Any type of stop and go driving beats down the average real quick. In the summer, with unblended fuel, my mileage was around 21 for the same type of driving.
<edit> There's a lot of info on fueling boxes available on the forum and can be accessed by a search, but you should be looking at gauges and transmission upgrades if you intend on adding a box. Boxes usually make short work of the stock tranny.