The South Dakota Board of Pardons and Paroles has an application for executive clemency online in PDF format. You may complete and submit that petition, as an application for a pardon.
"Upon the granting of a pardon, the Governor shall order that all official records relating to the pardoned person's arrest, indictment or information, trial, finding of guilt and receipt of a pardon shall be sealed."
You can miss defenses and say the wrong things if you are not represented by an advocate as your attorney. Also, I do not know how the laws of the Indian Reservation might impact the pardon request. I have never done that before. The lawyer you retain would have to do legal research to get the up-to-date answer for you.
You might find my Legal Guide helpful "Ethics: Yes I Need a Lawyer!"
You need a lawyer. Check with a lawyer in your locale to discuss more of the details.
Good luck to you.
NOTE: This answer is made available by the out-of-state lawyer for educational purposes only. By using or participating in this site you understand that there is no attorney client privilege between you and the attorney responding. This site should not be used as a substitute for competent legal advice from a licensed professional attorney that practices in the subject practice discipline and with whom you have an attorney client relationship along with all the privileges that relationship provides. The law changes frequently and varies from jurisdiction to jurisdiction. The information and materials provided are general in nature, and may not apply to a specific factual or legal circumstance described in the question.
Are you saying that you were convicted in FEDERAL court of the felony, resisting a federal officer? You might contact the Federal Public Defenders office in South Dakota (link below) or a private attorney in your state to determine whether the state clemency proceeding could provide any assistance to you. I tend to think not, but I cannot advise you as I do not practice in South Dakota.
As a matter of federal law, it is illegal for a felon to possess a firearm. 18 USC 922g.