We present a technique of passive microrheology based on tracking micron-sized particles which are denser than the surrounding fluid and therefore they sediment into a quasi two-dimensional layer. Theoretical corrections for the diffusion coefficient of polystyrene, magnetic, and melamine resin microspheres, situated at different average heights above the bottom of the container cell, allow to unify the corresponding results for the zero-shear viscosity and the dynamic modulus on low-density fluids, such as low-concentration mixtures of glycerol and poly(ethylene oxide) in water and 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid. Limitations of the experimental and data analysis methodology are discussed by developing experimental-like simulations of Brownian fluids.