While Chinooks definitely vibrate, I find it odd that you were having as much as you decribed at those rather low cruise speeds - we used to cruise at 120-130 kts regularly in CH-47C & D models, and faster if in a hurry & light.
Didn't start getting annoying until 140+ kts in most with the current composite blades if a decent track & balance was done, the C models when they still had metal blades were even smoother.
Wonder what the differences were between the military & civilian models were that could account for that, or was it just poor track & balance?