Is this a fuel filter? Or?
 

1988 E350
5.8l Gas (Above 8500 GVR engine)
Auto, No A/C

Notice the inline fuel filter located in the background!

What is this (leaking) canister thing for?

http://i394.photobucket.com/albums/p...ister_Base.jpg

http://i394.photobucket.com/albums/p...l_Canister.jpg

That is the fuel pump reservior. It is supplied by the in-tank pump and provides a more immediate fuel supply to the main fuel pump which is about 2 feet forward and just above the main fuel filter. I had to replace mine at about 180,000 miles. It was clogged up and I ended up breaking the case in the process of unscrewing it. Good luck resealing it although I wouldn't recommend it. This is one of those parts that you may have to buy from a dealership. At least the single tank version is reasonably priced. Here is the page in the Ford parts online catalog: FordParts.com Econoline#&&/wEXAQUHaGlzdG9yeQUEdHJ1ZbeJQwrDnn1brKWLK+LH7viyyDA r

PN 9K044 for gasoline single tank models.

If yours was clogged and/or full of dirt, as mine was, seriously consider checking the condition of the in-tank pickup strainer and the in-tank pump as well. These pumps are very sensitive to ingested dirt, sand, rust, etc. and can be destroyed if the strainer has any holes in it. Since the strainer rests on the bottom of the fuel tank, it can be prone to abrasion wear from vibration, surface rust and time. I also flushed all the fuel lines; supply AND return. Fortunately, the fuel injectors don't seem to be contaminated.
Check everything in the system and keep it as clean as possible. I hope this helps.