We present radio observations of the afterglow of the bright gamma-ray burst GRB 980703 made between 1 day and 1 yr after the burst. These data are combined with published late-time radio measurements and existing optical, near-infrared, and X-ray observations to create a comprehensive broadband data set for modeling the physical parameters of the outflow. While a wind- stratified medium cannot be ruled out statistically, it requires a high fraction of the shock energy in the electrons and so is not favored on theoretical grounds. Instead, the data are consistent with a fireball model in which the ejecta are collimated and expanding into a constant-density medium. The radio data cannot be fitted with an isotropic shock but instead require a jet break at åisebox-0.5ex 3.5 days, not seen at optical wavelengths because of the presence of a bright host galaxy. The addition of the full radio data set constrains the self-absorption frequency, giving an estimate of the circumburst density of ni̊sebox-0.5ex 30 cm$^-3$, a value that differs substantially from previous estimates. This result is consistent with the growing number of GRB afterglows, for which broadband modeling yields nrs̊ebox-0.5ex =0.1-100 cm$^-3$, with a typical value of rae̊box-0.5ex 10 cm$^-3$.