When I discovered the problem I thought it must be a misalignment in the way the brackets were mounted to the fuselage, but the assembly is still very stiff even after it's removed. I need to look at the assembly and fitting procedure in the manual to understand how it's supposed to work, but it sure looks like the bolts/nuts that attach the torque tubes to the brackets are too tight.
Like the flaperons, fixing this will wait awhile. Primary focus for now is getting the fuselage ready to go to the welder for tail reinforcement and additional mounting tabs for avionics and antennas. The disassembled rudder torque tubes will go as well, for gusseting.