I've had this same situation twice in AZ and have successfully reclaimed my property from the neighboring lot both times (two different properties).
I know it's not what you'll want to hear, but your first step may be to have your lot surveyed and the corners marked (assuming there is not a historical survey that you can find). Additionally, I'd have them delineate the line in question while they are out there.
Make sure that your surveyor records their survey map with the county recorder (which is required by law btw) and also verify that s/he does not have any active complaints w/the state (seriously, there are a few very bad surveyors out there) before you lay down the 1,000 bucks or so.
Once you have your corners and the p.line in question marked you can either try and work it out w/the neighbor yourself or get with a lawyer who specializes in such.