Use jQuery on your page to sort the divs.
Ideally you want your divs to all have the same CLASS so you can identify which divs to randomize.
I’m not familiar with the thesis theme but presumably there is a testimonials.php or something similar you could modify. If so, add your jQuery in there.
Here’s someone else answering the same question:
https://stackoverflow.com/questions/1533910/randomize-a-sequence-of-div-elements-with-jquery
I hope that helps.
You should be able to put your jQuery at the bottom of the testimonials.php file. The only thing you’ll have to change from the code in the link above is to insert the correct class name for your divs.