Shear strains, which change the donor wave functions, greatly affect impurity conduction, which depends sensitively on the wave-function overlap of neighboring impurity states. Using uniaxial compression along [111], we investigated as a function of stress the critical impurity separation ${d}_{c}$ for the transition from non-metallic to metallic conduction and impurity conduction in the intermediate concentration range, $7\ifmmode\times\else\texttimes\fi{}{10}^{16}<N<3\ifmmode\times\else\texttimes\fi{}{10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. The largest stress applied was 2\ifmmode\times\else\texttimes\fi{}${10}^{9}$ dyne ${\mathrm{cm}}^{\ensuremath{-}2}$. The main effect of stress is a change of the activation energy ${\ensuremath{\epsilon}}_{2}$ of impurity conduction. In arsenic- and phosphorus-doped germanium, [111] compression decreases ${\ensuremath{\epsilon}}_{2}$ and increases ${d}_{c}$. In antimony-doped germanium the opposite is observed; compression increases ${\ensuremath{\epsilon}}_{2}$ and decreases ${d}_{c}$. At 1.2\ifmmode^\circ\else\textdegree\fi{}K, [111] compression can increase the resistivity of antimony-doped germanium by a factor of ${10}^{7}$. Using the same orientation and temperature, a decrease of the resistivity of arsenic-doped germanium by a factor of 5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ is observed. These results suggest that (i) the activation energy ${\ensuremath{\epsilon}}_{2}$ depends strongly on the wave function overlap, and (ii) shear strains change the donor wave functions originating from the individual valleys by an amount which is proportional to the valley-orbit splitting of the donor element.