Im new to Flexigrid.. but faced same situation and used this code.. its working for me.. im hopeful that your problem can be solved..
var query = $("#fltr_county").val();
var data = {"groupOp":"all","rules":[{"field":"fltr_county","op":"ew", "data": '"'+query+'"'}]};
$('#flex2').flexOptions({
filters : data,
qtype : "admin_client_contract_id",
query : query,
}).flexReload();