What Causes Terrible Lower Back Pain And Stiffness In The Upper Legs?

I have terrible lower back pain and stiffness in my upper legs at the joint. My knees hurt and my neck. However, I am most concerned when I sit or lie for a while my leg or legs have a wave feeling to come over them and shake for a few seconds. Then a few minutes later it returned. Last night I took a valium and that seemed to make this subside although it is now 5:OO PM AND I CAN TELL it is returning. Never occurs when walking or standing. Can you tell me what might be causing this strange sensation? Thank you very much.