  We present the results of a 21-cm HI survey of 27 local massive gas-rich
late-stage mergers and merger remnants with the Robert C. Byrd Green Bank
Telescope (GBT). These remnants were selected from the Quasar/ULIRG Evolution
Study (QUEST) sample of ultraluminous infrared galaxies (ULIRGs; L$_{8-1000 \mu
m} > 10^{12}$ L$_\odot$) and quasars; our targets are all bolometrically
dominated by active galactic nuclei (AGN) and sample the later phases of the
proposed ULIRG-to-quasar evolutionary sequence. We find the prevalence of HI
absorption (emission) to be 100% (29%) in ULIRGs with HI detections, 100% (88%)
in FIR-strong quasars, and 63% (100%) in FIR-weak quasars. The absorption
features are associated with powerful neutral outflows that change from being
mainly driven by star formation in ULIRGs to being driven by the AGN in the
quasars. These outflows have velocities that exceed 1500 km s$^{-1}$ in some
cases. Unexpectedly, we find polarization-dependent HI absorption in 57% of our
spectra (88% and 63% of the FIR-strong and FIR-weak quasars, respectively). We
attribute this result to absorption of polarized continuum emission from these
sources by foreground HI clouds. About 60% of the quasars displaying polarized
spectra are radio-loud, far higher than the $\sim$10% observed in the general
AGN population. This discrepancy suggests that radio jets play an important
role in shaping the environments in these galaxies. These systems may represent
a transition phase in the evolution of gas-rich mergers into "mature" radio
galaxies.
