The distribution of individual molecular contributions to the second-rank rotational correlation function is introduced and used to construct probes of heterogeneity in rotational dynamics. The ideas are tested in a molecular dynamics simulation of supercooled liquid CS2. Both the quantity of heterogeneity and its lifetime or exchange time tau(ex) increase as the temperature is lowered through the supercooled state, and increase strongly as the mode-coupling temperature T(c) is approached. Crossover from Arrhenius to super-Arrhenius behavior of the rotational relaxation times tau(1) and tau(2) is observed, direct evidence of fragility in CS2. The T dependence of tau(ex) is stronger than that of the rotational times, and it may approach them from below at T(c), although the simulation is then very difficult. A detailed characterization of other aspects of the dynamical crossover is obtained, and the general implications of rotational heterogeneity for supercooled dynamics are discussed.