Problem solved, sort of. I changed out the stepper driver board, and now the Z-axis is moving well. However, somewhere in the process, my y axis has started missing steps. I'll post on that in a bit, most likely. The belt is tight, the skips are audible, and the pulley set screws are tight. When I watch the belt and pulley, they move and stop together, even with the missed steps. If I start a print of a test cube, the bottom layers look great, and then Y starts missing when going to the more acceleration-intense wall buildup. Gonna double-check Vref, but might play with maximum acceleration a bit.