1968 Dart sudden rear end noise.

Just replaced the fuel pump.yesterday abd decided to take the dart out. After driving for about 15 minutes the car felt like it was pulling a trailer and I heard a sound like a marble clattering around in the rear end or possibly a brake drum. I was driving it home and I'd hear a scraping,clicking sound when the car would go into 3rd (torqueflite) and every once in a while there would be a sound like a marble bouncing or rattling around. I just got it home and haven't pulled off the drum to.see if a brake spring broke. Anyone here have any ideas? I'm really depressed as I've done a lot of things and wanted to make this a daily driver since my past darts were super reliable. This one is fighting me. Lol