Sounds like the Sherman gearbox is doing what my old ZTZ gearbox did. The final drive gear is press fit on the output shaft that mounts the sprocket on some of these gearboxes. The knurling on the output shaft can wear down and then the gear literally just spins on the output shaft under load. I fixed the gearbox by drilling and tapping a set screw in the gear collar and drilling a shallow hole on the output shaft to mate with the set screw. At least that's what I remember doing. You could pin the two together with a roll pin by drilling a hole straight through the final output shaft and the gear collar.

Others might have an opinion of which gearbox is better as I don't think I have either one.