The rotational alignment of ${h}_{11/2}$ quasiparticles has been observed in the even-even $^{138}\mathrm{Sm}$ and $^{134}\mathrm{Nd}$ nuclei by means of heavy-ion fusion-evaporation reactions. The structures observed are compared to those found recently in $^{136}\mathrm{Nd}$. The ground-state bands of the $^{136}\mathrm{Nd}$ and $^{138}\mathrm{Sm}$ isotones (N=76) possess significant triaxial shapes (\ensuremath{\gamma}\ensuremath{\sim}-25\ifmmode^\circ\else\textdegree\fi{}). Rotational alignment of ${h}_{11/2}$ protons induces a shape change towards the prolate (\ensuremath{\gamma}=0\ifmmode^\circ\else\textdegree\fi{}) shape, while ${h}_{11/2}$ neutron alignment induces a change towards the oblate (\ensuremath{\gamma}=-60\ifmmode^\circ\else\textdegree\fi{}) collective shape. In the more deformed $^{134}\mathrm{Nd}$ nucleus, band crossings are observed in both the ground-state band and the \ensuremath{\gamma}-vibrational band. The crossing in the ground-state band is attributed to ${h}_{11/2}$ protons, while a second band that feeds into both the ground-state band and the \ensuremath{\gamma}-vibrational band is built on the [${h}_{11/2}$${]}^{2}$ neutron-aligned configuration. In addition, negative-parity sidebands built on two-quasiproton configurations were seen in these nuclei. The systematics of the proton and neutron alignments are discussed.