Car body "weaving" around

I've got a 99 OBS w/ 134k on it. In the past 6 months I replaced a lower ball joint and got the car realigned. It has been doing this weird weaving anywhere from 40-70 mph where the body weaves back and forth. The steering wheels also vibrates and I can feel very slight left and right periodic tugs of the wheel, but are not in synch with the body movement. The tires were all aired up to spec prior to driving and have even tread, wheels are straight. I jacked up the car and checked all the ball joints which are fine.

Please point me in the direction of what to consider or check next, as I'm kinda stumped by this one.