St. Johns County?
The database lists a site 3 in your area.
Florida Power & Light Trunking System, Various, Florida - Scanner Frequencies
There may be other FPL sites in monitoring range.
If you have not done already, I'd start with programming a site/system for FPL into your 396XT.
If you want to try Unitrunker, you can configure the program to accept control data from the radio. The program will display call activity, LCN numbers, and site number on screen. You must provide frequencies.
The program can't tell one EDACS system from another so each time you start monitoring an EDACS site, it creates a new temporary system. Manually type in the control channel frequency (highlighted in red). If the system was previously logged, the program will switch to that system. To keep your radio from wandering off to monitoring other systems or conventional frequencies, you must program it to follow a talkgroup on the system (preferrably one that isn't actually used) to keep the radio on the control channel.
I know - this is a lot of hassle. The PSR500 is much easier because you can place it in tune mode and key in a control channel frequency. Sorry, the Unidens just don't work that way.