Its the shaft that drops down from the gearbox. It's hardened steel. It has hole/pin at each gear, the weak links. Shaft sheared at the lower gear & cracked up to the 2nd gear in the gearbox. No gears or bushings were damaged. The pins are even ok.
Was not hard to spin the jackscrews by hand to get the jack retracted. If the jack somehow bound up I would think it would start clutching rather than shear/crack a shaft.
I think I might take the original gearbox to a shop & see what it would cost to have a hardened shaft machined.