Power window has a very hard time going up, but ok going down.?

Only on the driver's side of my F-250. It 'chugs' at going up, not a slow pace but 'chug chug chug...' and if I grab it and pull it up it sorta does alright. Goes down just fine, but I guess that's gravity for you. But it isn't a fast sliding down, it's a normal speed. I'm thinking it's the motor, but I wanted some more ideas before I went that route, and hopefully it isn't the motor!