Steve, Welcome to the forum~!
Really I should leave this to the gear heads but..I know with hood open on the drivers side rear u can see the mount and if u pulse the gas while in drive, with foot on brake, while someone is up front observing from the side, it should be solid without cracks and not more than an inch of movement aprox.
If the mounts are solid, I wonder if a P1811 long adapt shift code, could also cause this.
Last time my PA was on a lift, I checked the frame bushings and made sure bolts were tight and looked for cracks in frame bushings..all is well there on mine though.
1997 Buick Pk Ave (Soft Ride) Suspension!