You can use the jQuery based client-side API of jqGrid.PHP to get/set selected rows on the client with javascript (no postback).
First, obtain a jQuery reference to the grid's client-side object using:
Then, once you get the ID, you can perform a lot of operations against it, for example to select a row, just pass the row key
jQuery('#grid').jqGrid('setSelection','1014');
To get the selected row, you can use:
var selr = jQuery('#grid').jqGrid('getGridParam','selrow'); // returns null if no row is selected