I bought a whole H2 rear for $200 or $250, just for the e-locker. I hung on to the axleshafts and disc brake parts for years (to convert the rear to 8-lug), but scrapped all of that when I moved. So I've sent 2 pairs of 8-lug shafts to scrap and I'm now looking for another. I think I can make 2000's stuff work and I do see those axles come up once in a while around here. I have disc brakes that I made on the back, but never had a parking brake. They're basically 1/2-ton front brakes.
So yeah, I think if you had the time and space, you could make some money, but the biggest opportunity is probably still buying whole trucks. I once met a guy who made his living on nothing but buying and parting out GMT800 era trucks.