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I'm with Cary on this one. Wedding vows should not be taken lightly, and, I may offend some with this statement, but I think the LW's wife is doing just that.
A marriage means sharing your dreams and making compromises. I think being voluntarily away from one's spouse for two years, when the other objects, is selfish ... it would be different if she were in the Peace Corps while she was single and they still wanted to be together inspite of the distance.
However, she wants to go into the Peace Corps without her husband's blessing and yet keep the marriage intact. It's as if she married him under false pretenses. It is not fair to the husband. Two years apart so early in the marriage was not in the realm of possibility for him when he took his marriage vows.
The wife, if she is serious about her marriage, ought to consider short-term options for working abroad. Or, she ought to find a way to work with a non-profit/foundation in their metropolitan area that will give her the opportunity to go abroad for short trips.
On the other hand, if the Peace Corps is the only option she is willing to consider, then she will just have to accept the consequences of her actions. If that means letting her husband go, then she should do that.