When people get sod, they often think that all they have to do is water and fertilize and it will look great like it did when it was first put down. The thing that you need to remember is that sod was grown on very good soil, then cut off and put on top of your soil. It's roots then grow into this new environment. If your soil is poor, imbalanced, too acidic/alkaline, etc. then the grass is going to struggle. It's a bit early for a synthetic spring feeding, but Milorganite would be fine. I suggest a comprehensive soil test to determine the health of your soil. Since you are in Minnesota I assume the sod was Kentucky Bluegrass. KBG shouldn't require overseeding as it spreads, but only if it's happy. Here are a few questions that will help others determine what your problems could be.

What kind of grass is your sod? What is your watering schedule/fertilizing schedule? Any shade?

Also some pictures of the bad and good areas would be a big help.

Also while you get that answers, here is some fertilizer reading for you that reinforces Tiemco.

Fertilizer 101
http://www.garden-counselor-lawn-care.com/lawn-fertilizers.html
http://www.garden-counselor-lawn-care.com/fertilizer-numbers.html
http://www.garden-counselor-lawn-care.com/chemical-fertilizer.html

Fertizlier Calculator:
http://aggie-turf.tamu.edu/aggieturf2/calculators/fertsheet.html

Thank you tiemco and oceanfrank. Appreciate your help here. I'm not exactly sure what kind of grass it is - I'm assuming Kentucky Bluegrass. I try my best, as needed, to water couple times a week for about 45 minutes (set on timers). I don't have underground sprinklers. I've tried my best to fertilize typically about early May, August, October (winterizer) timeframes. I'm no expert on fertilizing, but just trying to do something to help lawn. I have trees planted, but they are small. So no major shade at all. Lawn typically goes dormant look pretty bad in hot August months. Also, my soil deep down under is mostly clay material. Not a lot of black dirt, just underneath sod area. Now where can I get a soil test? I've never performed a soil test before. I have photos, but am not sure best way to attach to this post right now. Again, thanks for you advice here.

Sounds like you are watering too often. Water as infrequently as possible. 1" once a week is best if your grass does not tell your otherwise. Put out cups to capture water that have 1/4" marks on them. Keep track of the time and when the level reaches 1", that area is done for the week. And now you know how long to water each section going forward.

Use Photobucket and post the HTML for pictures

Home Depot, Lowers, or Amazon will have inexpensive and less accurate soil testers, but useful if nothing else available. You can find a local extension college that can perform a more accurate test.

Thanks oceanfrank. I've included the current photo of my lawn. Any thoughts/recommendations going forward? I will try to do soil test, but what exactly will this tell me to do? Just trying to understand. Thank you again.

Here is a link that might be useful:

Go to www.loganlabs.com. Read about how to take your soil sample, and mail it to them with a check for $20 for their basic soil test. Do not buy one of those do it yourself tests, they are pretty worthless.