What is the box like ???
If its slack, noisy, sometimes difficult to change, I would strip it and check it over.
Being an 80 it should have had an easier life than if it was a 100, but 70,000 is a respectable mileage (unless its been re-furbed before of course).
Other jobs . . Check the rear suspension / swingarm. bevel box bearings.
There'll be other ideas along soon
Bob.