[REDNAX]

The post recommending questions posed on specific Ford Diesel boards is the correct answer. Commercial vehicles are spec'd to a different standard than light-duty pickup trucks. They are expected to survive the abuse of low-paid, unskilled drivers and city traffic, with potentially dozens of engine starts per day. And merging into and out of 35-45 mph traffic (and your vehicle sounds about right to me, given your descriptions of its performance) is what it was made for.

One of those three boards, and maybe three or four members will be able to help you determine the best course of action. I have had some experience with this, and know that there is no substitute for experience. It may be that some trans mods will work (personally, I'd NEVER have a full manual valve body on a street vehicle), and certainly it sounds as if some diagnosis is in order.

In the meantime I would get ahold of the service manual -- drivetrain -- for that vehicle and start checking out the simple stuff: linkages, fuel pressure, restrictions that can be done easily from the engine compartment or underneath.

On the other hand, the many box-trucks I've driven (rented by the boss or me) tells me that fuel economy changes are somewhat unlikely, but ensuring factory specs in all areas are dead-on might alleviate some of the fuel burn under certain circumstances.

Reliability trumps pocket-change mpg "improvements".