We have a 1992 F250 7.3L longbed 4x4, which gets 10 to 12 mpg. I'm sure this can be attributed to the 6" lift and larger (though not HUGE) tires.
Hm, you should be doing better than that. This is 460 gas motor territory. I get that in my F250 and it is only 2 years older than yours. Especially given that it is a base F250, not a "heavy duty" which is different from "base" F250. As for tires, larger tires should lower your RPMs, resulting in a better mileage when empty. (See
http://www.ring-pinion.com/calculators.shtml )
You consider using the standard tires that came with it, to keep the default 3.73 rear axle ratio for this diesel engine. I thought about getting a different rear end for my truck (go from 4.10 to 3.73 or even 3.55) until someone told me that 460cid engine needs 4.10 rear. It is something about it getting its max torque/HP at higher RPMs. Your diesel didn't even come with anything lower than 3.73.
If you have overdrive in your transmission, you should definitely be doing better than that. I am not a diesel person (in fact, I don't know that much about gas trucks) but it seems something is wrong. I hear the standard things to try are:
Oxygen sensor
fuel filter
Spark plugs and wires
Basically a tune up.