Services_Digg2 is just a client library in PHP that forwards all calls to the API and the API does support searching by urls. What you need to pass is the md5 hash of the url instead of the actual url. PHP has a md5 function that you can use to get the md5 hash of a url.

The API call then would be for story.getAll and you pass the previously calculated md5 hash as a parameter link_hash.

http://services.digg.com/1.0/endpoint?method=story.getAll&link_hash=a23658a0828e2fb388b7c83f61e235e6

This hash above is for the URL http://www.nytimes.com/2010/01/05/health/05docs.html and the response from the API is:

<stories timestamp="1262740374" total="1" offset="0" count="1"> 
 <story link="http://www.nytimes.com/2010/01/05/health/05docs.html" submit_date="1262729293" diggs="70" id="18288654" comments="6" href="http://digg.com/health/For_F_D_R_Sleuths_New_Focus_on_an_Odd_Spot" promote_date="1262739603" status="popular" media="news"> 
  <description>Look closely at Roosevelt’s portraits over his 12-year presidency. In his first two terms, there is a dark spot over his left eyebrow. It seems to grow and then mysteriously vanishes sometime around 1940, leaving a small scar.  </description> 
  <title>For F.D.R. Sleuths, New Focus on an Odd Spot</title> 
  <user name="leaprinces" registered="1227624484" profileviews="23186" fullname="Princess Leia" icon="http://digg.com/users/leaprinces/l.png" /> 
  <topic name="Health" short_name="health" /> 
  <container name="Lifestyle" short_name="lifestyle" /> 
  <thumbnail originalwidth="190" originalheight="126" contentType="image/jpeg" src="http://digg.com/health/For_F_D_R_Sleuths_New_Focus_on_an_Odd_Spot/t.jpg" width="80" height="80" /> 
  <shorturl short_url="http://digg.com/d31EjiI" view_count="192" /> 
 </story> 
</stories>

The story element in this response has an attribute called diggs which is what you need.

To get this through the PHP library, the code should look something like:

$url = "...";

$api = new Services_Digg2;
$stories = $api->story->getAll(array( link_hash  => md5($url)))->stories;

foreach($stories as $story) {
    // I am not sure how to access the diggs attribute
    // Maybe this works. If not, just
    // var_dump the $story object and 
    // see how to access the digg count information.
    echo $story[ diggs ];
}