Good Morning Dave, 

Thank you for contacting the U of M Extension Service. 

Oak wilt could be diagnosed by viewing the leaf canopy in the summer. The fact that the bark is missing, and there are woodpecker holes in the trunk, makes me think there is an insect infestation that is drawing the woodpeckers to peck at the bark, making it fall off. 

Here is a link to the U of M Extension Yard and Garden website about oak wilt: https://extension.umn.edu/plant-diseases/oak-wilt-minnesota

If you scroll through the information, the details expand. 

Here is another excellent link about the drought affects on trees in MN: https://extension.umn.edu/natural-resources-news/drought-minnesotas-woodlands

Good Luck!