I have had to deal with this before. 1st thing to know, is let them know, that it is a structural issue and you may or may not be able to fix the shaking to an acceptable level.

The whole reason for the shaking is because the beam itself is vibrating (otherwise it wouldn't do it).
1st thing to do is remove the rubber pad - the way you describe you have it mounted, is making the situation worse. It needs to be bolted solid against that beam.
2nd, get another arm to attach from another (not the same) beam, a little further down the pole. This will help stabilize the assembly. You can use a stabilizer arm from a DirecTV sat dish for this, if you like and if it will reach. Also, you can wrap your rubber pad around the pole, where this 2nd arm attaches. Here in this location, it will actually help in mitigating vibration. Just make sure where the 2nd arm is attached to at its base, is solid and 'no' rubber pad there.

[edit] Lowhz posted while I was typing. That Chief piece looks nice, wish they would have had that around 10 years ago when I dealt with that Infocus projector.