We have investigated the effect of Zn-doping on the superparamagnetic relaxation of nanosized nickel ferrite particles. Our temperature-resolved AC-susceptibility data demonstrate that the relaxation of these systems is well described by an Arrhenius-type law, behavior which allows a precise determination of the effective energy barrier to magnetization reversal, Ueff. We observe that the Zn-content dependence of Ueff exhibits a pronounced maximum when half on the magnetic Ni2+ ions are replaced by nonmagnetic Zn2+. The average blocking temperature (measured at a given frequency) shows a similar behavior. The observed increase of Ueff is attributed to an enhanced magnetic anisotropy induced by the crossover between two magnetocrystalline regimes. Our findings demonstrate the ability to control the superparamagnetic relaxation of ferrimagnetic spinel nanoparticles through chemical manipulations.