2002 Saab 9-5 Creak

I have had a 2002 Saab 9-5 for almost a year now and up until now, there have been no major problems with the car. I have even driven it across the country once (Indiana to California) and I had no problems. Recently, there has been a creak that sounds like it is coming from the right rear tire. I took it into a mechanic and he initially thought that it was the bushings. After checking the bushings, he found that they were ok and he told me that he couldn’t isolate the sound. The sounds increases in consistency and volume at high speeds and when I go over bumps in the road. I am beginning to think that it might be a shock. Any help would be much appreciated because I plan on driving it back across the country this Summer.