Just wanted to thank everyone for thier inputs!
My gut feeling is that the rear end is worn. I already replaced the front and rear shocks, and there are no alignment adjustments on rear suspension. I've already taken it in for an alignment. This has been an ongoing problem since I've owned the car. As long as I do not take it on long trips...it's fine. The car sits normally and still rides nice...so it's difficult to visually see a problem.
Since I have recently moved to Northern California (san francisco)...I'm still in search of a reasonable and trustworthy mechanic.