In this work we studied the two-dimensional ionization structure of the circumnuclear and extranuclear regions in a sample of six low-z ultraluminous infrared galaxies using integral field spectroscopy. The ionization conditions in the extranuclear regions of these galaxies (~5-15 kpc) are typical of LINERs as obtained from the Veilleux-Osterbrock line ratio diagnostic diagrams. The range of observed line ratios is best explained by the presence of fast shocks with velocities of 150-500 km s-1, while ionization by an AGN or nuclear starburst is in general less likely. The comparison of the two-dimensional ionization level and velocity dispersion in the extranuclear regions of these galaxies shows a positive correlation, further supporting the idea that shocks are indeed the main cause of ionization. The origin of these shocks is also investigated. Despite the likely presence of superwinds in the circumnuclear regions of these systems, no evidence for signatures of superwinds such as double velocity components is found in the extended extranuclear regions. We consider a more likely explanation for the presence of shocks, the existence of tidally induced large-scale gas flows caused by the merging process itself, as evidenced by the observed velocity fields characterized by peak-to-peak velocities of 400 km s-1 and velocity dispersions of up to 200 km s-1.