my newer 99' LS is also doing this (older 97' LS did this with 255k, was bad lower control arm bushings) when reversing and turning and coming to a stop. I did have an alignment done where they had to loosen the strut rod bushing or adjust for caster. I remember having to tighten the LCA bolts about a 1/4-1/2 turn. But it seems to have come back (I don't drive this car much at all anymore since I got a Honda that's a bit easier on gas and uses regular)