DRG THE LANDING ROLL, ABOUT 4000 FT DOWN THE 5002 FT RWY, THE LEFT MAIN LANDING GEAR COLLAPSED & THE ACFT VEERED OFF THE RWY. AN EXAM REVEALED THE BELLCRANK FOR THE LEFT MAIN GEAR RETRACTION LINKAGE ASSEMBLY HAD FAILED FROM OVERLOAD, WHICH ALLOWED THE GEAR TO COLLAPSE. THE REASON FOR OVERLOAD WAS NOT DETERMINED. THE PLT'S LAST MED CERT WAS DATED 6/6/83.