7.25 axle splines

Sorry my dyslexia kicked in and I thought the broken gears came from the 72-rear axle.
So basically, you need to change ring and pinion with the spider gear case from the 72 into the 73-rear axle, I have not done that yet, maybe someone here has, or
since you have both there maybe take them both apart test fit and let us know.
or what Charrlie S said.