How do I mobilize the Lumbar Joints?

I found out my problem.

My Lowest Vertebrae (sp?) Is moving, but the 3 above it aren't, thus whenever I stretch only the bottom one moves, so Ive got a stiff lower spine, with the bottom joint moving only.

Last time my physio gave me a massage kind of thing that mobilized the joints and i felt right as rain, but it stiffened up again.

What stretches or exercises can I do to get the joints moving?

the Mckenzie stretch/cobra doesnt work as only my bottom joint moves inthe stretch, and the top 3 stay stiff, they arent fused or anything, just very ... very stiff.