The dynamics of individual components in 1,4-polyisoprene/poly(vinylethylene) miscible blends are studied using two-dimensional deuteron exchange NMR. The rate of the backbone reorientation process near the glass transition is quantitatively determined for each species in a miscible blend as a function of temperature. We demonstrate that the broad glass transition arises both from a wide distribution of segmental motional rates for each species and from intrinsic differences in the motional rate between the two species. In addition, the temperature dependence of their motional rates in the blend DSC glass transition region suggests that the two components undergo distinct effective glass transitions, which is consistent with previously observed thermorheologically complex behavior. The origins of dynamic heterogeneity are examined further by comparing the experimental results with a simple model calculation that takes into account the effect of composition variations in an ideal miscible blend. This comparison suggests that the observed dynamic heterogeneities can be explained only by including two distinct contributions : local composition variations in the blend and intrinsic differences in chain mobilities.