Our 9 month old female is very nervous of large trucks when we walk. What can I do?

I meant to add this question to my last email training question....our 9 month old female Giant Schnauzer has developed a fear for traffic and loud trucks. Because we live in rural area, we rarely see traffic except for UPS or FED EX trucks. As of late, I have been taking her to our quaint downtown Main Street for walks in hopes of making her more comfortable with the loud noise. From the time we've had her at 4 months, we have expanded her "socializing" her via various situations such as car rides to town, road trips, kids, etc. so I'm not sure where this is coming from. Any suggestions? Am I making it worse by putting her in that situation? There hasn't been any direct negative contact with trucks to make her afraid that we are aware of. I do use lots of praise and ample amounts of treats during our walks downtown. Most dogs chase trucks, ours avoids them to the point where she almost starts to shake. I'm hoping she "grows out of it." Thanks for your thoughts.