Illustrative embodiments of tool extensions and methods of using such tool extensions are disclosed. In at least one illustrative embodiment, a tool extension may comprise a drive core configured to transfer rotational torque from a first end to a second end opposite the first end, where the first end is configured to be removably coupled to a tool to receive rotational torque from the tool, the second end is configured to be removably coupled to a fastener to supply rotational torque to the fastener, and the drive core is bendable between the first and second ends. The tool extension may further comprise a shell surrounding the drive core and containing an electro-rheological (ER) fluid configured to transition between a flexible state in which the shell permits bending of the drive core and a rigid state in which the shell resists bending of the drive core.