  Dislocation assemblies exhibit a jamming or yielding transition at a critical
external shear stress value $\sigma=\sigma_c$. Nevertheless the nature of this
transition has not been ascertained. Here we study the heterogeneous and
collective nature of dislocation dynamics within a crystal plasticity model
close to $\sigma_c$, by considering the first-passage properties of the
dislocation dynamics. As the transition is approached in the moving phase, the
first passage time distribution exhibits scaling, and a related peak {\it
dynamical} susceptibility $\chi_4^*$ diverges as $\chi_4^* \sim
(\sigma-\sigma_c)^{-\alpha}$, with $\alpha \approx 1.1$. We relate this scaling
to an avalanche description of the dynamics. While the static structural
correlations are found to be independent of the external stress, we identify a
diverging dynamical correlation length $\xi_y$ in the direction perpendicular
to the dislocation glide motion.
