We have talked to a few appraisers and the say landlocked land is significantly cheaper than market value
That doesn't make any sense. Market value is what a willing buyer would pay to a willing seller. Market value is (or should be) based on the quality and characteristics of the land. The market value of a lot that is landlocked should be less than the market value of a lot that isn't landlocked, bit it's still market value.
Somebody is either confused or BSing you.
Try this for shits and giggles. Look up your lot on Zillow. See what the Zestimate is (yeah, I know, but this is for shits and giggles, remember). Then look at the Zestimates for the other lots in the area and compare the value per square foot (or per acre) between landlocked lots and non-landlocked lots. Better if you can find lots of either or both types that have sold in the past 12 months.
You might also talk to several real estate agents, pretend you want to sell, and ask for a market analysis. You'll get that for free.
If your relationship with your ex is cordial see if she would agree to fixing a price with the above information.
What are we talking here? Thousands, tens of thousands, hundreds of thousand?
Do you have the cash to pay her in full? If yes, and she needs or wants cash now, you are in a strong position to negotiate a favorable outcome. Money talks.
If she doesn't want to put an end to this tell her to pick her favorite realtor and you'll sell the property and split the proceeds with her. Then there won't be any argument about how much the property is "worth."