Julian of Eclanum was a major player in the Pelagian Heresy, and is
constantly mentioned in relation to his debates with Augustine. One
piece of information I can't find though is where Eclanum is. It's
clearly in Italy, but either it doesn't exist anymore, or its name has
changed, so what is the modern equivalent? I'd really like to know
exactly where Julian comes
from.

Hello.

The ruins of ancient Eclanum (also known as Aeclanum or Aeculanum) are
in modern "Mirabella Eclano."

The town is on the ancient Via Appia in the province of Avellino in
the region of Campania.

From an Italian web page:
"...Giuliano di Eclano (oggi Mirabella Eclano, vicino Avellino)..."
Translation: "...Julian of Eclanum (today Mirabella Eclano, near Avellino)...'
http://www.santamelania.it/approf/agos_ostia.htm

Here's a map that shows Mirabella Eclano's position relative to Naples:
http://www.asmenet.it/opencms/opencms/asmenet/mirabellaeclano/conoscere_arrivare/?folder=/asmenet/mirabellaeclano/conoscere_arrivare/

Here are some photos of ancient ruins of the town:
http://xoomer.virgilio.it/davmonac/sanniti/aeclanum.html
http://spazioinwind.libero.it/popoli_antichi/altro/Aeclanum.html

Also see: Nodeworks Encyclopedia: Aeclanum
http://pedia.nodeworks.com/A/AE/AEC/Aeclanum/

Also see: Classical Gazeteer:
"Eclanum (AEculanum), a town of the Hirpini, Samnium, near Calor fl.,
R., on Via Appia, bet. Beneventum (15) and Sub Romula (21). Grotte di
Mirabella."
http://www.ancientlibrary.com/gazetteer/0144.html

---------------
search strategy:
eclanum -julian
aeclanum 
"giuliano di eclano"
etc

I hope this helps.

Great, more than enough information, and pretty quickly too. Thanks!