A strong worded letter by a lawyer is probably a good investment. You have shares and demand to know what happened to them.
I also asked around and heard through the grapevine that the startup was sold for less than liquidation preference. But I don't know how reliable this is.
I suppose my main hesitation in hiring a lawyer is that it would cost me a bunch of money, only to find out that the startup was sold for less than liquidation preference. If I did get a lawyer to send a "strongly worded letter", would the company be required to provide details about the acquisition? Given that my co-founder is not being cooperative, I suspect he wouldn't share anything that he isn't legally required to
A drawn out lawsuit would be expensive, but asking a lawyer how to get clear information about what's going on shouldn't be prohibitively expensive.