Can a propagating spin–2 mode participate in a local thermodynamic process without assigning an autonomous entropy or a unique local energy to the wave itself? We show that this occurs as a local thermodynamic work process in Jordan-frame scalar–tensor gravity, where a timelike scalar gradient defines an internal gravitational thermodynamic medium. Building on the first-order TT deformation–response relation, we identify the local shear power ${𝒲}_{\varphi }={\pi }_{ab}^{(\varphi )}{\sigma }^{ab}$ from scalar-frame traction mechanics and show that its leading pure-TT contribution is second order, gauge invariant, and enters the exact scalar-sector energy balance. We further show that the same effective anisotropic stress controls the scalar–tensor modification of the tensor-friction term, so the local shear-work process is directly encoded in gravitational-wave propagation. In geometric optics the corresponding ratio of shear work to GR-normalized tensor energy is controlled by the running effective Planck mass, providing a local thermodynamic interpretation of the standard modified gravitational-wave luminosity-distance law. Finally, we extend this shear-excitation mechanism to nonminimally coupled scalars, metric $f(R)$ gravity, and viable luminal Horndeski theories, and identify the condition required for the same interpretation in more general tensor sectors.