If this is her rear leg she may have injured her knee area making her limp. A cruciate ligament tear will do this as will a partial tear. You can read about this problem here
www.marvistavet.com/html/ruptured_anterior_cruciate_lig.html
If it is a ligament issue often surgery is the only fix.
If its a front leg or not a knee issue then it could be the dog has broken or cracked a toenail which is really painful. You can see if the dog reacts to you wiggling each nail - if one is sore you won't be able to miss that it hurts. You or the vet or a groomer can cut the nail off and stop the bleeding with styptic. This hurts the dog when you cut it but feels lots better once its all over with.
If you can't determine the problem as its been going on so long you may want to see your vet as it could be a broken toe or other injury that would require splinting is what is wrong.
Hope this helps you!NHolmes38583.4200397801

Have you checked her elbow? dogs can have hereditary weakness in the elbow area that will cause limping and in the shoulder joint area too.
You can read about elbow dysplasia here
http://www.upei.ca/~cidd/Diseases/musculoskeletal/elbowdysplasia.htm
and about osteochondritis in the shoulder area here
http://www.thepetcenter.com/xra/ocd.html
She could be straining a ligament in her leg or even have a neck issue that is causing pain from a nerve that travels down into the foot area.
The fact its the same side all the time makes me suspect its a joint issue.
You might find using glucosamine chondroitin such as Cosequin or Glycoflex will help over time. Might be worth having the vet check too.
I know the dog I had with the shoulder issue they put a cortisone shot in one shoulder joint that solved the problem for her.