Hey Fred,
Those gearboxes sure are messed up. My god there are a gazillion parts in there.
As it stands, there is nothing servicable in gear box. If you replace anything, get whole kit. Buying just gearbox or top motor costs as much or more than whole kit. It is Electric right?
If the gear box is damaged, check drive unit play. Anything over 100 thou will stress the gearbox. When radial rings wear in a 7400, the upper plate contacts the moveable bottom....steel on steel contact. Just look under at the drive tire towards top of drive unit, If you see a wear ring, radial rings are shot!