You never did say if it was a stick or auto?
Do you think the rear is located where it belongs while accelerating, or decelerating?
Does it do it under any deceleration at all? Like, is the force of the rear shifting directly proportional to the speed at which you pull off the gas pedal? (i.e. slowly ease off the gas= little shimmy/abruptly disengage throttle=big shift)
Does it sit level?
How is the wear on the tires, any strange patterns?
Have you measured your wheelbase from hub to hub on both sides to see if it's square?
Are you getting any noises? Clunks, grinds, buzzes, etc.
How old is the suspension?
Did this just drop out of the sky, or right after working on it, or was it a long time coming?
What drive line parts have you changed from stock, from the clutch out to the rear tires? (one thing on my mind is adjustable control arms)
Have you checked your shock mounts?
How are your engine/tranny mounts? (doesn't sound like them, but I've heard at least 100 very different symptoms that all turned out to be mounts). Gimme some more!