We present results on the commissioning of the two-module RadiaBeam / SLAC dechirper system at LCLS. This is the first installation and measurement of a corrugated dechirper at high energy (4.4 - 13.3 GeV), short pulses (< 200 fs) and while observing its effect on an operational FEL. Both the transverse and longitudinal wakefields allow more flexible electron beam tailoring. We verify that, for a single module at a given gap, the strength of the longitudinal wake on axis and the dipole near the axis agree well with the theoretical values. Using direct longitudinal phase space mapping and X-ray FEL spectrum measurements we demonstrate the energy chirp control capabilities. Show Partial Abstract