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I would think the logic should be a loop up to the peek jump point like so:

if(Keyboard.GetState().IsKeyDown(Keys.Space) && !jumping) {
    float peek = 64f;
    while(_position2D.Y != peek) { _position2D.Y += 1; }

Personally, and this is just me, I would move the variable peek out to a more publicly accessible point so that it can be easily edited/referenced. That is just me though.