The cop is accused of shooting and beating the unarmed cab driver in Huntington Station after a night of drinking.
The grand jury is expected to consider charges ranging from assault to attempted murder.
DiLeonardo denies any wrongdoing.
Published reports say DiLeonardo escalated a roadside dispute with Thomas Moroughan before shooting him on February 27, 2011.
A doctor who treated DiLeonardo after the shooting reportedly noted he was slurring his words, smelled of alcohol and was hostile. DiLeonardo refused to provide a blood or urine sample.
DiLeonardo and his partner, Officer Edward Bienz, remain on duty.
The Internal Affairs findings were issued July 31, 2012. The report didn't say whether the officers should be criminally charged and whether departmental charges it recommended were ever brought.
Assault charges were dropped against Moroughan in 2011. DiLionardo had initially said the cab driver tried to run them down, so he shot Moroughan twice through the windshield.