We have measured fully momentum-analyzed recoiling target ions and scattered projectiles, produced in ionization of He and H2 by 75 keV proton impact, in coincidence. The momentum of the ejected electrons was deduced from momentum conservation. From the data we extracted fully differential ionization cross sections as a function of the polar electron emission angle (for fixed electron energies) and as a function of the electron energy (for fixed electron emission angles). Comparison between experiment and various distorted wave calculations confirms that under kinematic conditions where the post-collision interaction plays an important role, the few-body dynamics underlying the ionization process are still poorly understood.