Online estimation of 3D ground reaction forces (GRFs) is becoming increasingly important for closed-loop control of lower-extremity robotic exoskeletons. Through in-verse dynamics and optimization models, 3D GRFs can be used to estimate net joint torques and approximate muscle forces. Although instrumented footwear to measure vertical GRFs in out-of-the-lab environments is available, accurately measuring shear GRFs with foot-mounted sensors still remains a challenging task. In this paper, a new outsole-embedded optoelectronic sensor configuration that is able to measure biaxial shear GRFs is proposed. Compared with traditional strain-gauge based solutions, optoelectronic sensors allow for a more affordable design. To mitigate the risk of altering the wearer's natural gait, the proposed solution does not involve external modifications to the footwear structure. A preliminary validation of the outsole-embedded sensor was conducted against validated laboratory equipment. The test involved two sessions of treadmill walking at different speeds. Experimental results suggest that the proposed design may be a promising solution for measuring shear GRFs in unconstrained environments.