We describe a photodetector array based on photoconductance-monitoring by four-point probing. This detection scheme is aimed specifically at detecting changes within a speckle or microscopic fringes within a larger nonuniform optical intensity distribution. One specific application is the detection of lateral displacements of these speckles or fringes, for example, in laser light reflected from optically rough vibrating surfaces. With a prototype, we have detected subnanometer surface displacements interferometrically. We also demonstrate speckle-based, noninterferometric detection of a guitar body's vibrations at a standoff distance of 6 m with nanowatt power. We observe and explain the prototype's limited frequency response by considering space-charge effects. This detection scheme is most useful in low-power, low-frequency applications.