Yes, this can be done if you are confident in your abilities. Some bottle jacks and thick posts/beams to distribute the load, maybe some comealongs to help pull the walls in. Check to see how much sag there is in the rafter itself.
Check your email James....Yeah, I think you are correct Eric. Very good point. Sometimes I miss the forest for the trees.
Yikes .As a side note, if you do try to raise it for some reason, you can pull the walls in some while jacking the ridge. If is complicated to do without hurting somebody though.
If you do it, DO NOT use bottle jacks. We used too and always checked for plum. That didn't matter one day. The bottom of the 4x4 shot out, or the jack tipped, or something. The 8' 4x4 went through 2 walls removing one stud and bouncing off another wall and came back into the room we were in. Somehow, it went between two of us going one way and then between two others coming back. No, we could not follow it with our eyes. We only had the skid marks and holes in the walls for clues. They move fast when they feel the need, I guess.
We only use floor jacks now.