My 05 250 4x4 with a 6" lift and 37s would get 13-15mpg. SCT Livewire 4" exhaust and innovative tunes. But that was in Central Texas at around 1100' above sea level and driving like a jack a$$ regularly.
My truck stayed at 16 mpg most of its life only running a performance bully dog tune. My mileage started dropping off fast when front end parts started wearing out. It got to 12.5 mpg before I found problems. The front end unit bearings both were hard to turn with the truck jacked up. My bearings werent even making a lot of noise like you think a wheel bearing would. Give them a check.
Another thing to mention....I lost one of the new unit bearings within 15K miles. My knuckle seal was shot on the driver side, which allowed dirt and mud into the spindle bearing inside the unit bearing. If you're already that far into a bearing change, with those miles, might want to change em.
The hub assembly/unit bearing is just a little more than a brake job. Pull the caliper, rotor, then 4 bolts from the rear of unit bearing.
The knuckle seal is just behind that. The axle will pull out with the assembly off. Knock the old seal off and put the new one on.
The only trick to that is making the knuckle seal tool. I'll try to find a link showing how to make it. Only requires a short piece of black iron pipe and a 3 or 4 inch floor flange.
You can test the knuckle seal with a vaccuum pump from the connection in the wheel well before you pull the unit bearing. The seal is one of the three seals needed for ESOF 4x4 to work if hubs are in auto.
ford part# for 99-04 is f81z-3254-cb and usually around $32 bearings are less than 200 from rock auto in auto extra brand. also check u-joints to make sure they are not stiff causing adverse steering, also lisle sells a seal driver specifically for those seals
The link above has another link to making the tool. It costs less than $10 to make and only tool required to make it is a grinder. About 18" of black iron screw pipe, a floor flange and cap. Knocks the seals right on.