If there is already a divorce then your appeal window is very soon to close or has closed already .
If you failed to bring up an issue during the divorce you may be too late to bring it up on appeal.
Whether or not you can successfully challenge a post nup due to a change of circumstances is beyond me --even if door is open --which it may not be.
Short of a gun to your head or him twisting your arm its hard to argue you were forced to sign a post nup! Some of his threats seem pretty nasty and the court might see your side --but not if its too late.
Your sole hope is in fast review by the best attorney you can find...