  We report on a study of the 2300-2600\AA FeII/FeII* multiplets in the rest-UV
spectra of star-forming galaxies at 1.0<z<2.6 as probes of galactic-scale
outflows. We extracted a mass-limited sample of 97 galaxies at z~1.0-2.6 from
ultra-deep spectra obtained during the GMASS spetroscopic survey in the GOODS
South field with the VLT and FORS2. We obtain robust measures of the rest
equivalent width of the FeII absorption lines down to a limit of W_r>1.5 \AA
and of the FeII* emission lines to W_r>0.5 \AA. Whenever we can measure the
systemic redshift of the galaxies from the [OII] emission line, we find that
both the FeII and MgII absorption lines are blueshifted, indicative that both
species trace gaseous outflows. We also find, however, that the FeII gas has
generally lower outflow velocity relative to that of MgII. We investigate the
variation of FeII line profiles as a function of the radiative transfer
properties of the lines, and find that transitions with higher oscillator
strengths are more blueshifted in terms of both line centroids and line wings.
We discuss the possibility that FeII lines are suppressed by stellar
absorptions. The lower velocities of the FeII lines relative to the MgII
doublet, as well as the absence of spatially extended FeII* emission in 2D
stacked spectra, suggest that most clouds responsible for the FeII absorption
lie close (3~4 kpc) to the disks of galaxies. We show that the FeII/FeII*
multiplets offer unique probes of the kinematic structure of galactic outflows.
