  We experimentally investigate the stochastic phase dynamics of planar
Josephson junctions (JJs) and superconducting quantum interference devices
(SQUIDs) defined in epitaxial InAs/Al heterostructures, and characterized by a
large ratio of Josephson energy to charging energy. We observe a crossover from
a regime of macroscopic quantum tunneling to one of phase diffusion as a
function of temperature, where the transition temperature $T^{*}$ is
gate-tunable. The switching probability distributions are shown to be
consistent with a small shunt capacitance and moderate damping, resulting in a
switching current which is a small fraction of the critical current. Phase
locking between two JJs leads to a difference in switching current between that
of a JJ measured in isolation and that of the same JJ measured in an asymmetric
SQUID loop. In the case of the loop, $T^*$ is also tuned by a magnetic flux.
