Tough one, Alison.
To summarize some of the other great comments here, there are two issues:
1. Voting control of company
2. Current and potential $$ value of the shares
If your co-founders are approaching you because they have a problem with #2, then they are not your friends at all. You took a risk, you earned a compensation package, and that's all water under the bridge. If you feel guilty, like you are going to net $10M for surfing Facebook for 3 months in your parents garage while your co-founders sweated out an amazing company, that's a different story. But I suspect you aren't "that founder".
It sounds like the situation is #1. They need more shares to distribute, but don't want to dilute their voting rights. I would talk to a lawyer about the options here, but some sort of a share conversion, or a swap of voting shares for non-voting shares sounds plausible. I've never heard of "renting" the voting rights, but that's intriguing as well, and sort of forces them to keep on good terms with you to earn the right to vote, essentially on your behalf.
Another option might be to put you on the board (if you aren't already), and use your share block to augment their voting power. If everyone really is aligned on the future of the business, why are they so concerned about retaining majority control over you?
Lots to think about. But I smell some "win-win" possibilities here, don't give up on that yet.