Experiments were conducted during the NASA Equatorial Ionospheric Study (EQUIS) II sounding rocket campaign on Kwajalein Atoll to investigate the dynamics and stability of the postsunset equatorial F region ionosphere prior to the onset of equatorial spread F (ESF). Experiments took place on 7 and 15 August 2004, each composed of the launch of an instrumented and two chemical release sounding rockets. The instrumented rockets measured plasma number density, vector electric fields, and other parameters to an apogee of about 450 km. The chemical release rockets yielded lower thermospheric wind profile measurements. The Altair radar was used to monitor coherent and incoherent scatter in UHF and VHF bands. In both experiments, large-scale waves preceded the emergence of ESF depletions and irregularities. We propose that the large-scale waves were produced by a collisional shear instability. A three-dimensional electrodynamic model is constructed that can reproduce the shear flow observed by the rockets. On the basis of the model, we calculate the linear growth rate for the collisional shear instability and show that it can compete with the generalized Rayleigh Taylor instability. A scenario where the former generates the large-scale waves and initiates the latter is discussed.