It looks like it may be a prairie vole (Microtus ochrogaster) which has a gray belly ("or washed with whitish or pale cinnamon"). Esp if it was in open country. It occurs in the eastern half of the state. so seems to be within the range.
It could also be the long-tailed vole (M. longicaudus), but this species occurs in woody coniferous or brushy habitats and doesn't do the vole tunnel thing. It doesn't seem to be this however from what you say about the tunnels.
You also have the meadow vole there (M. pennsylvanicus), which is dull brown above with a gray belly. Can't rule this out.
Boulder is out of the range of the sagebrush vole (Lemmiscus curtatus).
The prairie vole seems most likely to me -- the tail is strongly bicolored in the 2nd picture, which also fits.
Thanks for the input, Chris, and for the head's up, Christian! Here are some of my favs of the bird- amazing that many times it was too close to fit in the frame as it flew by me while I stood on the shoulder of the road. The snowy shots are from today, others from last Saturday. Enjoy- Bill