We report the observation of resonant magnetotunneling between states of different effective mass derived from zone center ($\ensuremath{\Gamma}$) and zone boundary ($X$) points of the Brillouin zone in single-barrier GaAs/AlAs/GaAs $p\ensuremath{-}i\ensuremath{-}n$ heterostructures. Resonances arise in the conductance-bias characteristics for both Landau level index conserving ($\ensuremath{\Delta}n=0$) and nonconserving transfer ($\ensuremath{\Delta}n=0$). For $\ensuremath{\Delta}n=0$ resonances, structure with a splitting proportional to the difference in the cyclotron frequencies (${\ensuremath{\omega}}_{c}^{\ensuremath{\Gamma}}\ensuremath{-}{\ensuremath{\omega}}_{c}^{X}$) is observed, while for $\ensuremath{\Delta}n=0$ transitions a smaller splitting corresponding to ${\ensuremath{\omega}}_{c}^{X}$ is reflected. Analysis of these features enables the transverse (${X}_{\mathrm{xy}}$) effective mass in AlAs to be determined. Clear evidence for ${k}_{\ensuremath{\parallel}}$ conserving and nonconserving transfer in the elastic $\ensuremath{\Gamma}\ensuremath{-}{X}_{z}$ transfer process is reported.