I heard back from my friend in Rome.Here is a copy of his answer which probably doesn't answer your question (sorry).
"Colletti is the name of a philosopher/politic (Lucio Colletti), who lives in Turin. A lot of northern Italians are however of southern origins. I tried to find some Colletti in some search engines in Italian (www.virgilio.it or the italian versions of international engines like www.altavista.it and www.google.it). I found some Colletti in the zone where you come from (near Caltanissetta), probably they're your relatives..."
I hope this helps.I'll let you know if I learn anything else on this subject.