DRG FLT FM WILMINGTON, DE TO RUTLAND, VT, THE PLT DIVERTED TO BINGHAMTON AFTER RECG THE ALBANY WX & LEARNING RUTLAND WX WAS NOT AVAILABLE. AFTER LNDG, HE TOLD FSS PSNL HE PHONED RUTLAND & WAS TOLD THE WX WAS GOOD. HE GOT 2 WX BRIEFINGS BFR CONTG THE FLT & WAS ADZD OF FLT PRECAUTIONS FOR ICING & TURBC, AS WELL AS PIREPS CONCERNING ICING. HE FILED A FLT PLAN &CONTD ON TO RUTLAND. WHILE APCHG RUTLAND AT 6000', HE RPRTD MOD ICING. HE REQD CLNC TO CLB & WAS CLRD TO 8000', BUT HAD DIFFICULTY CLBG. HE REQD & RECD VECTORS TO BURLINGTON, WHICH HAD 3500' OVC & GOOD VIS. WHILE EN ROUTE, HE WAS UNABLE TO MAINT MIN SAFE ALT OF 5500' & DECLARED AN EMERG. AFTER DSCNDG BLO CLDS, HE SAID ICE WAS COMING OFF THE ACFT. HE RPRTD THE RWY IN SIGHT & WAS CLRD TO LND. WITNESSES SAID THAT ON FINAL APCH, THE ACFT LOST 150 TO 200 FT, THEN REGAINED SOME ALT SVRL TIMES. DRG 3RD LOSS OF ALT, IT HIT THE GND IN A FLAT ATTITUDE & SLID APRX 300' ONTO THE RWY. THE ACFT WAS NOT EQUIPPED FOR FLT IN KNOWN ICING CONDS. PITOT HEAT SW FND IN OFF PSN. DC-9 CREW RPRTD 15 KT LOSS OF SPEED ON FINAL APCH.