
Ditto has randomize parameter so I think you can do it with another Ditto call.
http://ditto.modxcms.com/files/snippet-ditto-php.html#randomize

[!Ditto? &tpl=`testimonials` &parents=`21` &randomize=`1` &limit=`1`!]
&display=`1`
If you want only one document to be displayed use this.is &display not supposed to be used with pagination?
&display=`1`