I had the same problem until I installed Kuchar filler plates. No more vibration. I highly recommend them. The way they are made they don't trap the dirt.
Once you engage the machine for the day, leave it engaged. The 9000 series are worse than the 20's for out of balance cylinders I swear. I have gotten in the habit of once it's engaged I leave it engaged till the end of the day. Even if I'm moving between fields and they are only a mile apart or so it stays on down the road. That is a Deere trademark and no real cure. I don't run filler plates because I only cut wheat and sorghum so the golf ball idea would be worthless............I really wonder how much good it actually does even though. If dust can throw that cylinder out of balance I would think golf balls in the wrong place could do the same. Spose Tiger Woods would endorce a Deere combineIJ *grin* lOl.
What crop are you having trouble inIJ If small grains whith no filler plates in, we usally have trouble with weeds and grasses building up on the back of the bars, in clean fields never seems to be a problem.
Replace your cylinder bars recentlyIJ Perhaps whoever installed them didn't put them in rightIJ They are paired off 1 left with 1 right angle ledges and they are a balanced pair.