I had this problem as well and could not figure it out either. I then installed Springs and swaybars on the car, re-aligned and the vibrations are gone.
Could be alignment?
AA Filter : BFGoodridge SS Lines : Challenge Sport X-Pipe : Vorschlag Poly Engine Mounts : TMS Subframe & Diff Bushings : KW S/O Kit : RDSport Sway Bars : MRF Eng Shaved Guide Supports : E36 Bumpstops : Mach Schnell UP : UUC SSK w/DSSR : ZHP Shift Knob : Evolve Tune Stg 2.
Needs install and swap -> TE37SL : Brembo BBK : Nitron NTR R3