Any "approaching quarter-century" GM without an entirely replaced front end suspension is long overdo. It could also be work done on the car, - miss-clocked coils or missing isolators, worn/missing shock cushions, rubbing exhaust on frame or drivetrain. Body cushions could be an issue - the only way is to get under and eyeball closely. Rear end same as front. Check loose gastank straps. Has the car ever been tweaked in a wreck? How old are the shocks anyway?
Basically, a million things, but sounds like susp. is best starting place. IDK ever hearing anything super exceptional with wagons v. any other B-/D-.
\'96SS SOB: SSRI, Herter Tune, Tri-Y II, , 3000 Edge, F/HO bars, METCO extendeds with CV MMC, Bilsteins, currently Vredestein Ultrac Sessanta (315/35 rear). Finally, wait for it... LT-4 knock module!
\'96FWB BUW-Butt Ugly White: PCMPerformance tune, HDFA w/ swissed filter, 1/2 SLP s.s. catback with H- and Flow Super 50s, oem opti rebuilt 3 times, SS Bars, METCO LCA, Airlift 1000, Monroe SS, PowerTrax No-Slip, CTS wheels with +1 rubber. Finally, yes again... LT-4 knock module!