A downhole stabilizer includes a radial expansion assembly deployed about, and configured to rotate substantially freely with respect to, a tool mandrel. The expansion assembly preferably includes at least one stabilizer block configured to extend radially outward from the mandrel into contact with a wellbore casing string. When deployed between uphole and downhole cones, the stabilizer block preferably includes a plurality of angled splines configured to engage corresponding splines disposed on the cones. Relative axial motion between the stabilizer block and the cones causes a corresponding radial extension or retraction of the block.