Based on information obtained from steady-state and time-resolved photoluminescence (PL) experiments done as a function of uniaxial stress, the symmetry of the lowest conduction band state for a series of type II Al0.15Ga0.85As/AlAs multiple quantum wells with a nominally fixed alloy thickness of 4.2 nm and AlAs thicknesses from 4.8 nm to 7.8 nm, is reported. Structures with AlAs thickness less than 6.2 nm have the Xz state (heavy mass in the growth direction z) as the lowest lying conduction band level while for thicker AlAs layers the Xx,y states are lowest in energy. For samples with Xx,y ground states, a time-windowing technique is used to simultaneously observe the PL from the Xz and lower-lying Xx,y states, providing a direct measure of the energy separation of excitons associated with the in- and out-of-plane X valleys in the AlAs. By studying the effect of the uniaxial stress on the time-resolved PL spectra, the X6 shear tetragonal deformation potential of AlAs, E2, can be estimated without complications due to the stress dependence of the valence band energy. The value of E2 obtained from two samples with different layer thicknesses is 5.8 +/- 0.1 eV.