UNLESS you can do a tension test to asses the integrity of the differential, you have to play it safe and assume that the CONE diff is worn out. I'd never buy a used cone diff without a test.
My axle guy puts the diff in a vise and locks one axle down, then tries to spin the other axle with a torque wrench. If the unit slips at anything under 80 ft/lbs, the unit is on its way out.
CONE differentials can be rebuilt but there are some drawbacks to that. It is because of this that even the factory suggested total replacement in the event of excessive wear.