Hi Heather,
One thing you don't want to do is increase the baking soda. Altitude probably is affecting the cookies. I don't know what the elevation is in SD -is it over 6,500 ft? Some things are affected at altitudes over 5,000 ft. and others over 6,500 ft.
Did you just move to SD? And if so, did the cookies just start going flat when you moved there? Or have you lived there and cookies came out fine until lately? Here is some information: Also, go to the website www.makegreatcookies.com
It is a great website about baking cookies and has information on baking at high altitudes, etc.
If the following suggestions or the website don't help, let me know and I will see if I can help you further. Pat 
1. Try decreasing the baking soda.
2. Don’t use a spread. Use butter or margarine (that is not made with cooking oil and water). Increasing the baking soda makes for a flatter cookie; decreasing it makes it have more shape. Increased baking soda = flat cookie.
3. Chill your batter.
4. Make sure that your baking sheet is not hot or warm. Let the sheet cool real good before using for the next batch.
High Altitude Cooking Recipe Adjustments (Altitudes over 6,500 ft.)
Baking Powder: Reduce each teaspoon by 1/4-1/2.
Baking Soda: Decrease as baking powder. No adjustments needed with acidic (fruit juice, ex.) ingredients.
Boiling Water: Allow an extra 10 minutes. Water boils much slower at high altitudes.
Fats: No adjustment unless substituting, then 1 cup shortening = 3/4 cup butter or oil.
Oven Temp: Increase 25 degrees F. For glass dish: decrease by 25 F.
Package Mix Cakes & Brownies: Add 1 tablespoon flour to mix (in addition to other high altitude directions on box.)