I'm rebuilding dana 80.
Found front pinion bearing, one closer to the pinion gear, inner bearing race moving around the pinion shaft. Obviously that is not good. but the question I have is.
Can I just replace bearing on the pinion or the pinion itself has to be changed as well??

 

If the pinion shaft is worn replacing the bearing is not going to fix it. If the fit is fairly tight (not turning it by hand) you may get away with loctighting it to the pinion.
Replacing the gears would be the best option.