We present complex admittance measurements on single-crystal yttria-stabilized zirconia and polycrystalline ${\mathrm{Li}}_{0.5}{\mathrm{La}}_{0.5}{\mathrm{TiO}}_{3}$ over the frequency range 5 Hz to 30 MHz and at temperatures ranging between 150 and 650 K. Electric-field relaxation in both fast ionic conductors can be described using Kohlrausch-Williams-Watts decay functions, but departures are observed at high frequencies and low temperatures. Electric modulus data obey the Dixon-Nagel scaling that has been proposed to be universal in describing the relaxation processes in supercooled liquids. Our data provide broader universality to the Dixon-Nagel scaling, and are interpreted in terms of the influence of mobile ions positional disorder on the relaxation dynamics.