Another poster mentioned bulging disc. The symptoms you described are exactly what my wife's symptoms were when it was discovered that she had disc problems. 20 years ago she started with numbness in her toes. It got steadily worse until she couldn't walk. An MRI discovered that she had one disc that was bulging and the one above it had actually ruptured and the "jelly" inside had run down the side of her spine and was impinging on her sciatic nerve.