Well, I feel much better now about getting off the horse after the bucking...
It seems that PAIN was the issue. I worked with her on the ground today (no intention of riding - just wanted to see if the hoof trimming made any difference) and she picked all four feet up for me with no problem, was very cooperative and willing, and showed a marked reduction in stiffness.
I am much more confident about sticking with this horse.
Thanks everyone for your advice!
Leasing a spoiled rotten trail horse...pretty - but a brat!