[thstone]

If I understand correctly, I want to clarify that the engine is backwards in a Boxster vs a 993 so the 993 motor or engine mounts are the same as Boxster transmission mounts. And yes, they are interchangeable between the 993 engine and the Boxster transmission.

You said that you used the 993 TT engine mounts - are these the stock 993 C2 hydraulic (fluid) units, the semi-fluid units from the C4, or the non-fluid/solid RS units?

I am going to guess that you used the semi-fluid or non-fluid mounts and if so, that is likely the source of the vibration. Swap back to the stock 993 C2 fluid mounts to see if this fixes the problem.