Disclaimer: I am not familiar with US law. Please see an attorney, and don't get worried before an attorney tells you to. :-)
I'm a little taken back by something no-one has mentioned yet. Share certificates. Over here, shares are registered to the owners name. For small early stage companies the share certificate might simply be a plain piece of paper, with a legal text that grants shares, mentions the nominal value and the series of issue etc, and is signed as required. In this case, the owners name is simply written on the share certificate itself, and and the company secretary keeps a record of all shares issued.
If you don't have a share certificate, then my gut feeling (without knowing US law) is that you don't have any shares at the moment? You have a contract that say you should be given shares now, but it hasn't happened yet. If the breakup was less than amicable, maybe there is a problem here.
So I'd advise you to contact a lawyer, identify the open issues, and send a warm an positive letter to the company, as Civil and Thoughtful suggests.
asking to be updated on current valuation and exit plans or just assume that their lawyers will contact me when an exit occurs?
Once the status of your shares is sorted out, I would say keep reading industry websites, and set up a google alert on the company name. The company is not required to send you ongoing newsletters of what is happening, their communication with the shareholders is mainly the annual report and the annual shareholders meeting. If an exit comes up, then there will generally be an offer to all shareholders to sell/exchange their shares at a set price, and I would expect you to be notified of this.
What are my rights as a shareholder, if any?
Mainly to vote at the annual shareholders meeting with the number of votes your shares give you, to receive pro rata dividends if the company chooses to pay out dividends; see this overview from Wikipedia.