the OE bushings should last upto 70K, even though some can drag longer and some will be gone before... regular bushes run for about $70-90 a pop, and I saw KO Performance selling them for about $25-30 a pop - so, there might be a spectrum in quality... the Poly bushes can run for $100-120 a bush...
the symptoms of the butt skiing, are consistent with the bad bushings. if the bushes are good but the symptoms are still there - then it could be another component of the suspension...

On a separate thought, check the threads on GUIBO and see if those symptoms are similar to yours - just to eliminate the guibo from this equation...