It depends. It's possible that he has adversely possessed that two feet (meaning he basically took it and you failed to act over a certain time period) and the neighbor now owns it. If he has not adversely possessed it, one remedy would be tearing down the fence.
This is pretty common in Montana. How much do you want to spend in lawyers to force him to move it? You may want to just talk to the neighbor first. Maybe he will compensate you for it, or maybe you two can work out splitting the cost of a newer and nicer fence for both of your properties. If you want to push the issue, you will need to hire someone to survey the property and make a determination before the fence comes down.
licensed attorney in Montana. Your specific state laws may be different. My answers do not create an attorney/client relationship. Only a signed fee agreement creates such a relationship. These answers are for informational purposes only.
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