You have many rights as a joint tenant including the right of equal access to the entire estate. Is that still the type of tenancy? It could have been severed. Do you have a partnership agreement? I think your first order of business is to obtain a preliminary title report to determine not only the present state of title but also the nature of any liens or encumbrances. Following that, or in conjunction with that task, you should retain an attorney to take appropriate action. The action you should take will depend on the exact circumstances and your desires. In most cases, a negotiated settlement is better than a lawsuit although you may have to file suit to stimulate those negotiations. I do not think this is something you can handle on your own for the simple fact that, in the last 2 years, you have been unsuccessful. Good luck.
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You have the right to partition and force the sale of the property. Note, your friend's actions are clearly hostile to you. If he is also paying the property taxes, the time for running of adverse possession most likely began at least two years ago. You need to protect your rights and act.
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