I think I may have found my (running) problem. Let me narrate the thought process:
I hooked up with my old training group and ran stadiums that night. I was taking short steps on the stairs, not bench to bench as I used to. I think running those steps and working on turnover may have clicked something in my head about how to relax.
So the next time I ran hill repeats at Workman Hill in Whittier, I started working on relaxation while going up the last climb. This translated quite quickly into a feeling of "tossing" with my foot by relaxing before I've even left the ground. The next hill workout, mostly on just the other side of Turnbull Canyon was an exercise in tossing the foot in the same way.
This worked with no soreness or tightness as a result. I'm super-optimistic.