Proton decay from the 3$\hbarω$ isoscalar giant dipole resonance (ISGDR) in $^{58}$Ni has been measured using the ($α,α'p$) reaction at a bombarding energy of 386 MeV to investigate its decay properties. We have extracted the ISGDR strength under the coincidence condition between inelastically scattered $α$ particles at forward angles and decay protons emitted at backward angles. Branching ratios for proton decay to low-lying states of $^{57}$Co have been determined, and the results compared to predictions of recent continuum-RPA calculations. The final-state spectra of protons decaying to the low-lying states in $^{57}$Co were analyzed for a more detailed understanding of the structure of the ISGDR. It is found that there are differences in the structure of the ISGDR as a function of excitation energy. ; Comment: Minor changes after review. Accepted for publication in Phys. Rev. C. 19 pages; 7 figures