A focal-plane detector for the Enge split-pole spectrograph at the Triangle Universities Nuclear Laboratory has been designed. The detector package consists of two position-sensitive gas avalanche counters: a gas proportionality energy loss section and a residual energy scintillator. This setup allows both particle identification and focal-plane reconstruction. In this paper, we will detail the construction of each section along with their accompanying electronics and data acquisition. Effects of energy loss throughout the detector, ray-tracing procedures, and resolution as a function of fill pressure and bias voltage are also investigated. A measurement of the 27Al (d,p) reaction is used to demonstrate a detector performance and to illustrate a Bayesian method of energy calibration.