I wouldn't rule out the possibility of the wheel bearings either. Either shot or improper tension for some reason. I second what Hyena said, tear it all apart and get a rebuild kit.
![Yes Nod :yesnod:](https://board.marlincrawler.com/Smileys/marlin/yesnod.gif)
especially if you don't know when the last time the knuckles were serviced.
![My Two Cents :twocents:](https://board.marlincrawler.com/Smileys/marlin/twocents.gif)
good luck. keep us posted.