Oscillation Freqency?
I am very curious as to the frequency of the lateral oscillation that is felt in the aft end of the 777. If the frequency is 1 Hz or less the associated mode probably does not involve flexing of structure. If it is 2 Hz or higher it very likely does involve structural flexing.
The 777 control laws involve functions designed to damp out oscillations at various frequencies, both lower frequency "rigid body" motions and higher frequency "structural mode" motions.