Scott and Steve a both right. The vehicle had very low value and sales of used cars are in fact an 'as is' sale. However, now that you have made a good faith effort to fix the things that were wrong, you should definitely get the lady to sign a 'Release' so that you can get on with life.
Your fact scenario is not clear as to who owned the truck, you or your aunt. Regardless, who ever was selling the truck had a duty to properly disclose those items that you knew were wrong with the truck at the time of sale. While I am not aware of any law that says you can not sell a vehicle with a broken odometer, a broken odometer definitely affects the value of a vehicle. She only agreed to pay $1,800 for the vehicle. She knew the truck didn't have much value. The mechanic who fixed the odometer should have provided you with a warranty. He should repair it. If the woman did something that damaged the odometer, she would be responsible for any repairs. Scott is right, though, it might be cheaper to work things out with her. What ever you decide to do, when you work it out, make sure you get a release in writing so you and your aunt will be done with this. You could always consult a consumer lawyer in your area.
The information provided should not be considered legal advice. I am not licensed to practice in any State other than SC. The results of your case will depend on the presentation of evidence, the law and other factors that may change depending on an in depth analysis of the facts of the case. Please see an attorney before making legal decisions.