i have seen this on my rdx's and other cars as well... someone explained it best in another thread when he said that no matter what people say, the "cvd"s in all rc cars are not exactly true constant velocity... as they turn, parts of it speed up and slow down, causing vibration. i think the closest would be the Losi dualjoint set....
i believe the SRC spool has the slots offset for this reason, as a comparison.
on the stand, try to rotate the diff halves in relation to each other, and see if it changes... if so, then like Korey mentioned, maybe set your car up to steer more, and dial back the travel...