Perhaps she's not a goner after all..

Following encouragement from my brother-in-law on the east coast who has a '75 Plymouth Valiant, I removed the driveshaft out of my Valiant and sure enough, the output shaft freely spun when it was put in drive and reverse.

The driveshaft is in normal condition as well. The spline looks perfect, except there's several dark marks on the part of the shaft right after the spline that's inside out the output shaft.

So would this be a result of a seized output shaft bearing or something else?