This SQL report shows a random message/reply in the forum system. It will only show the approved messages and the last revision.

To make sure the visitor gets a random post on every refresh, put the 'Cache Timeout' to 0 seconds. 

SELECT
  content
FROM
  Post,
  assetData
WHERE 
  assetData.assetId = Post.assetId
AND
  assetData.status = 'approved'
AND
  Post.revisionDate = (SELECT MAX(Post.revisionDate) FROM Post WHERE assetData.assetId = Post.assetId)
ORDER BY
  RAND()
LIMIT 1