Z-axis Stopped working

I've used my Stepcraft 2/600 successfully for several weeks now without a problem. Today I flipped it on its side to install a tool length sensor. When I put it back upright and powered it on, I found the Z-axis would no longer move. Thinking it was the wiring (no motor sound at all when I hit Z-), I removed the tool sensor wires and re-inserted/tightened the wiring block for the Z-axis motor. Unfortunately this didn't solve my problem.

At VERY low speed (high torque?) it mostly works (up and down) though still jams occasionally. Anything higher than around 10 though and it won't move at all, motor just hums. I removed the spindle mount and eased the screw by hand - it spins very easily. I also cleaned out the grease and loosened the bearings a bit to see if that would help. No joy.

When it does move I can see a bit of wobble in the bottom screw bearing at the spindle mount. So the screw's clearly no longer spinning straight. The wire length on the motor isn't long enough to remove the whole assembly as one, so now I'm trying to work out the best strategy for fixing what I'm guessing (hoping) is just a misalignment.

Suggestions?