Hi, would an axle from an 85 fury 8 1/4 rear fit in a 75 duster?

The "Fury" is probably a "Gran Fury".
Those were essentially the same as a Diplomat or a Fifth Avenue.
The width is fine. The leaf spring perches will need to be removed and welded back at 43" apart center to center.
Those cars that had these axles often had really TALL gears...Like 2.21, 2.45 or 2.76. The cost of a gear swap might kibosh the deal though.