This afternoon and evening, I've done quite a bit of monitoring of the DouglasCounty Simulcast subsystem and cannot find any indication of 851.56250 being part of the system.
Where did you run across this frequency? I didn't see it all afternoon on Pro96Com, nor on my UniTrunker which has been running since Wednesday of last week.
In fact, the system doesn't even make mention of an 866.56250, which would correspond to the re-banded frequency mentioned above.
I haven't had time to look at the license again, so that frequency could have been added to the FCC license, but not yet added to the system.
Time will tell!