Partial sacralization is a phenomenon which happens in few individuals. We call it as lumbosacral transitional vertebra where in the transverse process of last lumbar vertebra become spatulated, enlarged and fuses with the Sacral vertebra or ilium. This process usually takes several years and during this period it can cause severe low back pain. Once the segment gets fused it may not cause much pain...It is not a dangerous problem.
Next Steps
Consult the pain physician. Injecting a local anaesthetic and small dose steroid combination under X Ray guidance at the site where the two bones are about to join (pseudo-articulation) will be the right approach to relieve pain...you will also need a good therapist who will stretch and strengthen your back after the injection.