We present an experimental investigation of lossy and reactive shear forces at the nanometer scale using quartz-crystal tuning-fork shear-force microscopy. We show that this technique allows us not only to quantitatively measure viscous friction and elastic shear stress with a combination of high spatial and force resolution (better than 10 nm, and less than 1 pN, respectively), but also to obtain such quantities with the tip positioned at any arbitrary distance away from direct electrical tunnel contact with the sample surface. We are proposing that, even under vacuum conditions, the measured viscous and elastic shear stress (i.e., velocity dependent) are directly attributable to a third body filling the tip-sample gap. A simple model is given that allows us to obtain its local viscosity and shear modulus as a function of the tip-sample distance, showing that tuning-fork shear-force microscopy can be applied to quantitative analysis in nanotribology.