Code:
var data = [];
Ext.each(grid.getSelectionModel().getSelections(), function(rec){
  data.push(rec.get('id'));
});
Ext.Ajax.request({
  url: 'myurl',
  params: {
    id: data
  },
  success: function(){
    // do something
  },
  failure: function(){
    // do something else
  }
});