Can I obtain a divorce when spouse's whereabouts are unknown and we've been separated over 20 years?

Yes. There is a process for serving someone if you know his/her last known address. Get a lawyer, paralegal or the self help window at your court house to help you.

You can get the divorce. You will first have to do a required investigation to locate the spouse and then apply to the Court to serve the summons and Petition by publication in a newspaper.

Yes, you can obtain a divorce even if you cannot locate your spouse. After your file your Petition with the court, you must attempt find her and have her/him personally served with the paperwork. You must contact her last known residence, her/him last known employer, as well as her/his relatives and friends that you know of, also you should check Facebook and the internet. If all of those things fail, you can submit a request to the court to have your spouse served by publication (newspaper). Outline your attempts at finding her/him, and if the court believes you have done enough and she/he cannot be found, then they will allow you to serve by publication. Once that is completed the case can move forward without her/him.