SingleSelectCheckColumn is a CheckColumn extension that only allows one row to be checked at a time (so it behaves like a radiogroup).
Code:
Ext.ux.grid.SingleSelectCheckColumn = Ext.extend(Ext.ux.grid.CheckColumn, {
    onMouseDown : function(e, t){
        if(Ext.fly(t).hasClass('x-grid3-cc-'+this.id)){
            e.stopEvent();
            var index = this.grid.getView().findRowIndex(t),
                dataIndex = this.dataIndex;
            this.grid.store.each(function(record, i){
            	var value = (i == index);
            	if(value != record.get(dataIndex)){
            	    record.set(dataIndex, value);
            	}
            });
        }
    }
});