SHORTLY AFTER TOUCHDOWN AT MEMPHIS, TN, A SEVERE VIBRATION WAS FELT BY THE FLIGHTCREW IN THE COCKPIT. THE FLIGHTCREW THOUGHT THE VIBRATION WAS GENERATED BY THE RIGHT MAIN GEAR SO THEY HAD MAINTENANCE PERSONNEL CHECK THE RIGHT MAIN GEAR. NO ANOMALIES WERE NOTED. THE FLIGHT CONTINUED TO PITTSBURGH, PA, AND SHORTLY AFTER TOUCHDOWN, THE LEFT MAIN GEAR COLLAPSED. POST INCIDENT EXAMINATION OF THE LEFT MAIN GEAR REVEALED THAT TWO SPACERS IN THE TORQUE LINK ASSEMBLY WERE FOUND TO HAVE BEEN IMPROPERLY INSTALLED. THE INVESTIGATION ALSO REVEALED THAT DURING RE INSTALLATION OF THE TORQUE LINKS AND SHIMMY DAMPER AT THE LAST MAINTENANCE CHECK, THE TWO SPACERS WERE INSTALLED IN REVERSE OF THEIR NORMAL POSITIONS. ACCORDING TO FOKKER, REVERSAL OF THE TWO SPACERS WOULD RESULT IN AN UNDAMPED FREE PLAY BETWEEN THE UPPER AND LOWER TORQUE LINK RESULTING IN OSCILLATIONS IN THE LATERAL MODE OF THE LANDING GEAR. REVIEW OF USAIR'S AIRCRAFT MAINTENANCE MANUAL, SUPPLIED BY FOKKER, REVEALED THAT THE TWO SPACERS THAT WERE INCORRECTLY INSTALLED ON THE INCIDENT AIRPLANE WERE NOT DESCRIBED BY DIMENSIONS OR VERBIAGE.