You can have it "scrubbed" by a fuel company but if you fon't have too much of it, I'd dump it and clean out the tanl brefore putting in new fuel. If you don't have a tank access port, you can get a pipe fitted to a dshop vac thru the fuel gauge sending unit and vacumn it out that way (also you can remove the fuel with an oil change pump thru that opening).

Like Chuck suggested, you could have your fuel polished which would remove any contamination in the fuel.  I would have a concern about the sides and bottom of the tank. Seems like the experts like to have the tank half full so the can agitate the fuel to clean out the inside.  You could just pay them to pump out the tank and discard the old fuel.  It is then easy to remove the tank and have it cleaned out.  Several years ago I did that and had a radiator shop clean it for $50.  Then my tank was almost empty so I did not have a problem of finding lots of Jerry cans to store and remove the old fuel.  As for the "quality" of your fuel, I do not know if diesel itself goes bad with age.