Hi mtbguy,
Just from the description you gave its one of those ones that would be much easier to diagnose through seeing it and performing a full assessment. From what you said with the fact your back seized up, the pain stabbing pain in your gluts and ankle ties in with the fact your a keen cyclist. flexion at the knee and plantarflexion at the ankle coincide with myotomes L5-S1 and S1-S2 respectively and those are common repeated moves while cycling.
With cycling being apparently your main method to keep fit (unsure of your day job, do you work at a desk, have an active job?) and the description you have given, id suspect you have quite a stiff lumbar/sacral region and most of your movement probably comes via your hips or even Tx region. Id have a look at your SI jt and suspect that mobilizing Lx/Sx and following this up with rehab to maintain this would give you some relief.
Sorry its not very specific...advice is go and get yourself properly assessed.
Ross