Effects of static disorder and interaction with phonons on the dynamics of Frenkel excitons in molecular aggregates are studied by calculating the absorption of a weak probe in the presence of a strong resonant and off-resonant pump field. To second order in the pump amplitude, the self-energy which determines the Stark shift and dynamical broadening of the probe absorption is expressed in terms of the single exciton Green function and the two-exciton scattering matrix. For stronger pump intensities the self-energy is calculated using higher-order optical response functions of the system.