Nebraska plays a 4-3. The only 3-4 team in the big 12 last year was Kansas State, and they switched to a 4-3 this year. The Spread offense taking over that conference makes it extremely difficult to play a 3-4 there because the amount of time you spend in Nickel and Dime packages means you have to be able to play with a 4 man line.
I believe both Michigan and New Mexico State use 3-4 sets. In the two Michigan games I've seen this year, their base package was a 3-4 more often than not. And I think New Mexico State is a full time 3-4, but I'm not sure.