I'm going to have to disagree with orthodox I'm afraid, it's certainly not a main symptom.
SI / facet SYNDROMES may cause a dull achey (referred) pain down the leg in the pattern described; though you'd want to rule out neurology first, but not numbness. Classicly facet syndrome won't cause referred pain below the knee either, though I disagree with the text books on that.
Linked in with muscle imbalances, I'd suggest ruling out piriformis syndrome first up if the MRIs are clean (not just discs, but other cuases of neurological SSx); then I'd look at peripheral nerve entrapments.
If you can rule out those and trigger points, then I'd be inclined to go with SI/facet sydrome as a working diagnosis - and I'd say to keep checking to see if anything else comes to light after you've started treatment.
I have to say, your description sounds perfect for an L5 radiculopathy to me, so I'd want to be VERY clear that that is ruled out first.