I don't know where you are, but out here in the "Boonies" (Wentzville), MODOT plows I-70, I-64 and I61. We are in District 6 (St Louis District) and the only thing I could find in the RR Database is the MODOT District 6 - Mobile-to-Mobile frequency of 151.070 MHz.
As far as St Charles County, the RR Database only shows 2 frequencies for the Highway Department, 158.745 and 159.105 MHz.
As a (somewhat) interesting side note, I managed to get in front of one of the gang plowing teams one night and saw that the trucks use green lasers beams pointed down the road apparently to keep the proper spacing between the units. I guess this gang plowing thing is more of an exact science than I thought! As efficient as this method is for clearing the road, it is a real PITA when you are behind them... and late for work!