This should be a simple fix, but I just have not been able to find anything about it.
I am using both postData and editData to POST a variable to the server for
Inside your grid setup it would be:
postData: { KeyName: KeyValue },
You will see this extra parameter go out with your POST.
The example below will set the postData value, (if it was to change) and then trigger a reload of the grid.
$('#gridName').jqGrid('setGridParam', { postData: { UserName: userName }).trigger('reloadGrid', [{ page: 1}]);