lower back pain with no name

I am only 32 and in fairly good health. I have had three kids and no complications. After my last I starting having really bad back pain in the middle of my lower back which would come and go. It has been two years and has progressivly gotten worse. I have had x-rays which showed some scoliosis and my leg is shorter than the other. I have done some physical therapy which didn't help at all. It sometimes radiates into my kidneys. I had a ct scan which showed nothing. Now the pain is radiating down into my buttocks and legs. The doctors think it is a sciatic nerve and want to do a mri. If it were infact a sciatic nerve would I have felt the pain in my buttocks and legs a long time ago? I don't have insurance and not alot of funds to be throwing away on all of these tests. I am about pilled out. Is there anyone out there who can relate???