Last night I reworked all the communication to happen directly over serial, and hooked the leg up to a more capable power supply. Performance was greatly improved. Now the leg can max out the travel of the linear guide it's fixed to:
And the obligatory animated GIF:
That jump was actually still with a current limit set on the power supply. Turning up the supply to max current, the leg can easily crash into the hard-stop at the top of the linear rail. It actually managed to move the hard stop by a couple millimeters - I dialed back after that, because if the bearing manages to escape the rail, all the little recirculating balls will fall out.
I almost set it up for wireless control over XBee, but decided to take a nap instead. Wireless communication with a small LiPo battery fixed to the leg, and no cables dangling off would be beautiful.