The rear axle is solid, it moves up and down, but you can take the body and shift it over top of the axle to some extent because the shocks are mounted in a very soft rubber bushing. It seems to actually pivot because they are so soft, the rubber flexes, and the car moves back and forth. With a stiffer bushing, it will prevent most of this twisting. I know that the panhard bar infact obsorbs most of this movement that im talking about
And yes your right, the shock does work a little more, but its only minutely more. If anything, it feels more fluid. Heres a picture to help explain.
The arrows show movement from left to right, transfering from the tires, to the axle, to the panhard bar. Because the bottom of the shock has a bushing that will flex, as i have tested. When you install your new shocks, you can move the shock from side to side in the axle mount fairly easily just by hand, nevermind the weight of a car going around a corner at .5g. Now, from what ive realized, i could of infact upgraded the lower suspension bushing aswell, but i wanted to leave a little play there so not to stress/fatigue the mount. Anyways, with all that pressure into the panhardbar, the axle will shift from side to side under the body of the car, again as i tested with simple shaking excersises. Now you may say that the trailing arms actually limit any movement left to right, but again, the stock bushings are fairly soft and do move. I would like to upgrade these aswell with poly, but im guessing that it would be a personal thing, as i dont think there is a market for it.
With a very stiff suspension setup, that your already upset about how rough it is, im not going to recomend you upgrade these bushings because you might even hate your life a little more. If your ok with stiff suspension, and a stiff feel, as i am, then these are a great item.
Oh and to add, again, think of front suspension systems that have caster plates with no bushing at all. I know it is infact rough, but performance wise, its worth it, as i feel the rear bushings are.