Vibration when in Drive or Reverse

When I am sitting there with my foot on the brake and I have the car in either R, D, S, or 2... the car vibrates a lot. I hear a lot of vibration coming from the back of the car and the mechanic told me that it might be related to my aftermarket exhaust. I have a greddy exhaust and it is toching the chasis. could this be it, if so what can I do to avoid the vibration?
the timing belt was changed 10k miles ago, so I do not believe that it would be that.
I have tried putting a peice of high density rubber between the exhaust and the chasis, that helped it a lot, but there is still some vibration. I have a concern about this tho...
will this high density rubber melt? I do not know how how the exhaust can get after prolonged usage and I do not want it to melt.
thanx in advanced for any help