[Ronbo]

Sounds like one (or more) of the rear bushings has softened up and is deflecting under load. This could be either side depending on if it's the front or rear bushing. I guessing you have bushings instead of sperical joints since you list your cars as CSX's.

As Tom stated alignment could also be an issue, another remote possibility is the differential slipping on one side (you'd probably feel this as chatter though). Do you notice any clunks, shudders or other noises from the rear (granted these are rather hard to hear over the pipes and complaining tires)?

Easiest thing to do is get under there with a big pry bar and start tugging on stuff and see what's giving in, also look for bushing material squeezing out.

Dwight, what your describing affects solid axle rears, not independent.