The easment should be on her title/deed. she would have had constructive notice of it then.
short term solution pull the deed/title from the county and see if it exists.
Speak nicely to her and ask permission.
If it goes to court - it could take a significant amount of time and energy. You will most likely need an attorney.
The first thing you need to do is verify that the easement is "of record." Minnesota has two title record systems, registered and abstract, so you will need to check with the appropriate county office for property records and find out if the easement is of record. If it is, then you have a really strong case; if it isn't, then you will have a much more difficult case, but all is not lost.
Take the document and your title abstract to a local real estate attorney for review.
As my colleagues have indicated, you want to ensure that the document was filed prior to your present neighbor's purchase. If it was not, you may have a problem. You also want to ensure that the document is an easement and not a license. A valid easement would run with the land and would be irrevocable. A license is revocable.
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You should definitely consult with a local attorney in your area that has experience with easement disputes. He or she will confirm if the easement is "of record" and binding on the neighbor regardless of what the neighbor is now claiming. In my experience, because litigation is expensive and impactful for all parties, there is a high likelihood that a strong letter from your attorney warning of the consequences of interfering with your rights of access on the driveway will work and avoid litigation.