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We present continued radio and X-ray observations of the relativistic tidaldisruption event Swift J164449.3+573451 extending to $\delta t \approx 2000$ dafter discovery. The radio data were obtained with the VLA as part of along-term program to monitor the energy and dynamical evolution of therelativistic jet and to characterize the parsec-scale environment around apreviously dormant supermassive black hole. We combine these data with$\textit{Chandra}$ X-ray observations and demonstrate that the X-ray emissionfollowing the sharp decline at $\delta t \approx 500$ d is due to the forwardshock. Using the X-ray data, in conjunction with optical/NIR data, we constrainthe synchrotron cooling frequency and the microphysical properties of theoutflow for the first time. We find that the cooling frequency evolves throughthe optical/NIR band at $\delta t \approx 10 - 200$ d, corresponding to amagnetic field energy density fraction of $\epsilon_B \approx 10^{-3}$, wellbelow equipartition; the X-ray data demonstrate that this deviation fromequipartition holds to at least $\delta t \approx 2000$ d. We thus recalculatethe physical properties of the jet over the lifetime of the event, no longerassuming equipartition. We find a total kinetic energy of $E_K \approx 4 \times10^{51}$ erg and a transition to non-relativistic expansion on the timescale ofour latest observations ($\delta t \approx 700$ d). The density profile isapproximately $R^{-3/2}$ at $\lesssim 0.3$ pc and $\gtrsim 0.7$ pc, with aplateau at intermediate scales, characteristic of Bondi accretion. Based on itsevolution thus far, we predict that Sw 1644+57 will be detectable at centimeterwavelengths for decades to centuries with existing and upcoming radiofacilities. Similar off-axis events should be detectable to $z \sim 2$, butwith a slow evolution that may inhibit their recognition as transient events.

Radio Monitoring of the Tidal Disruption Event Swift J164449.3+573451. III. Late-time Jet Energetics and a Deviation from Equipartition