Just to be safe, I'm going to replace everything. None of the components are really bad but none are really good either, including spindles. I think the vibration may be the result of harmonics. Maybe it wasn't very noticeable with an old stretched-out belt because it would just slip but maybe during a certain point a few pulleys will be dragging at the same time and the new belt is grabbing a lot better.
The vibration is rather subtle and rhythmic. Sounds like when you hear a machine running in the distance for a long time and start to hear a rhythm to it and you tilt your head because you're not sure if you're actually hearing it or imagining it. When you're on the machine, you can feel it, and like I said in a prior post if you watch the top side of the deck the idler pulley (not the pivot one but the one that swings when you engage) will bounce up and down during the period of vibration. It's tight on its mount so it takes the arm with it. It's not constant though...it's rhythmic.
The deck is in wonderful shape and was able to get some of the components in "used-like new" condition. I guess that means the packaging is damaged but part is new. So they were like 1/2 price. I went with 2 new idler pulleys and 2 new spindles. I've already got new blades and a belt. Hopefully, this does the job.
Edit: And I wouldn't be surprised if the spindles are bent slightly. Not enough maybe for me to notice visually but I have heard the old blades hit the curb.