8.5 rear diff with an s-shaped spring is an Eaton and is rebuildable. Of the two-plate-and-coil springs versions, there are two types. If the case is a one-piece forging, then it is updated or aftermarket Eaton version- also rebuildable. If the case has a bolt-on end cap on the pass. end then it is Auburn and has cone clutches and is NOT rebuildable.
Other things to watch out for are whether case is two series or three series(2.56 and down or 2.73 and up) and axle spline count(28 or 30)