Hi Kevin,
there is mighty preSaveTrigger() JS function. Try put this code on your form (assuming that your_checkb_field is checkbox and your_field is the field you want to make required).
Good luck,
Jakub
function preSaveTrigger()
{
var frmMain = document.forms["main_form"].elements;
var checkb = frmMain["SET.your_checkb_field"].value;
if (checkb == 1) {
detailMakeReq("your_field");
} else {
detailMakeReq("your_field", true);
}
return true;
}