We probe the vibrational modes of the equilibrated self-trapped exciton (STE) in the mixed-valence linear chain material [Pt(en)(2)][Pt(en)(2)Br-2]center dot(ClO4)(4) using resonantly enhanced impulsive stimulated Raman excitation of the excited electronic state. In these measurements, excitons are created by photoexcitation of the optical intervalence charge transfer band, and after a delay to allow self-trapping and equilibration, the metastable STE is impulsively excited and probed within its red-shifted absorption band. The pump-pump-probe response reveals wavepacket oscillations at a frequency of 125 cm(-1) that are assigned to a Raman-active mode of the STE having Br-Pt-Br symmetric stretching character. This frequency is shifted from the 171 cm(-1) symmetric stretch Raman frequency of the ground electronic state, and from the previously observed 110 cm(-1) wavepacket modulation that accompanies the formation of the STE from the initially excited electronic state, reflecting a new component of the structural relaxation of the exciton.