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David, I replaced the control arm bushings and stabilizer bar bushings on my 64 98 many years ago with rubber components. I figured since a 98 is a luxury car there was no need for "performance" bushings. I've heard that the poly bushings can squeak; however, I don't know if this is an old wives tale or reputable feedback.
Poly bushings significantly reduce unwanted suspension motion, improving handling and response. This comes with the cost of degraded ride quality. Your call. If you choose poly, DO NOT use poly bushings in the front end of the upper control arms on the rear suspension. The GM four link suspension requires the upper arms to twist slightly when cornering. The more compliant stock rubber bushings allow this to happen. Poly bushings do not, and cause suspension binding as a result. It is acceptable (and relatively common) to run rubber bushings in the front of the upper arms and poly everywhere else. If you really want to eliminate the unwanted suspension motion, use the all metal ball joint bushings at the front of the upper arms.