First off, apologies for the non-fun situation.
"the other founders are asking me to return some/most/all of the equity that was vested up front."
Don't do this. Most likely this will be used to redistribute this share to themselves or the option pool. They WANT this potential share for themselves if the company is successful, and it's not in your interest to give it up.
You've started things, most likely, and if things were standard-ish, you might have been vesting your stock at 1/48 (or maybe 1/36 or something?) of the total each month (founder shares may also not sometimes not require a year's service to vest). You legally earned what you got during the 1/48 and have no obligation to return anything. The pre-vested up front stuff might have also been sizeable, and most likely will be a foundation for future work that this business produces.
The downside is your last comment about "may start a new company", which seems like they might not be above board. In this case you may need a lawyer, but it might not hurt to contact the board if you have investors on it. If there's no investment, you might have a harder position and you may (I am not a lawyer) need to watch them to see if any of those entities exit. In other words, you might have to sue to get what you are owed for the stock games. Most likely this is an empty threat, and you should probably indicate you are aware of your legal options and reserve the right to exercise them if this happens.
I don't have a great understanding of what happened there, but think about Facebook and what it has done with screwing initial employees from time to time, and they seem to have won at least in part.
Given, if the startup is already crippled due to not being able to work together, the chance of success at the end is reduced. Don't give up your shares - but don't count on them being a thing either.
They should be happy that you still have your shares because you have an interest in seeing them still be successful and are not ALSO going to form a company in the same space. Perhaps. Depends what your exit agreements were, if any. And you should be careful what you sign there as well.
But yeah, sounds like they are asking for things that are not in your interest.