We have fabricated and measured Josephson junction (JJ) defect spectrometers, which are frequency-tunable, nearly harmonic oscillators that probe strongly coupled two-level systems (TLSs) in the barrier of a JJ. The JJDSs accommodate a wide range of junction inductances, L-J, while maintaining a resonance frequency, f(res), in the range of 4-8 GHz. By applying a magnetic flux bias to tune f(res), we detect strongly coupled TLSs in the junction barrier as splittings in the device spectrum. JJDSs fabricated with a via-style Al/thermal AlOx/Al junction and measured between 30-45 mK with single-photon excitation levels show a density of TLSs in the range sigma(TLS)h approximate to 0.4-0.5/GHz mu m(2), and a junction loss tangent of tan delta(J) approximate to 2.9 x 10(-3). (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4744901]