It's time to drop the tanks and check/replace the sending unit/pickups. There is a plastic extension on the bottom of the pickup which contains a by-pass valve and the filter/screen. Unfortunately, this plastic part breaks off and/or the by-pass valve rots (rubber). It is there to allow the engine to get fuel if the bottom of the tank has enough water to freeze and plug up the normal pick-up.
If the plastic has broken off at the top where it attaches to the metal tube of the pickup (as both of mine were) you'll run out of fuel at about the 1/4 tank level. The new sending unit/pickup is a "Ford Stealership Only" item. Go here:
http://www.parts.com/partlocator/index.cfm?siteid=213787 after selecting your make and model, you need to look under "Electrical" to find the fuel sending unit. The 'units' are about $20.00 less at this site than at a "Stealership"($100.00+/-), but I don't know if "the shipping charges" will make it less in the long run. The front and rear units are different with different part #s. And even NEW sending units aren't very accurate. When/if you install new SUs fill the dry tank with 5 gallons of fuel and check what the gauge shows, add 5 more, check etc...
Rather than replacing the whole sending unit, some people have just added a diesel rated rubber hose with some kind of screen to end of hose. I'd tee it at the bottom of tank then reach to front and back. The original plastic screen does not have a fine screen, it's too hard to suck thickening cold diesel fuel thru a fine screen. There is a screen bypass valve on the upper part of the OEM plastic unit. If the bypass valve gets crap stuck in it or if it rotts away it will allow fuel in the line to run back into the tank..loosing the prime...allowing air into the system. P.s. I just checked on the above link yesterday and the price is $99.14
OkieGringo