THE PLT AGREED TO FERRY THE ACFT FOR ANOTHER PLT WHO WAS NOT IFR RATED & WHO HAD BEEN FORCED TO LAND AT KADOKA ON THE PREVIOUS EVENING DUE TO LOW FUEL & APCHG DARKNESS. IMC PREVAILED THRU-OUT THE AREA. THE COMMERCIAL/IFR RATED PLT DID NOT OBTAIN A WX BRIEFING OR FILE A FLT PLAN. HE TOOK OFF & CONTACTED ELLSWORTH AFB APCH CONTROL AT 1535 MST. LATER, HE RPRTD LEVEL AT 6000' MSL & SAID THE TOP OF THE OVERCAST WAS AT 5000'. WHEN TOLD THAT BOTH RAPID CITY & ELLSWORTH AFB WERE BELOW APCH MINS, HE ELECTED TO DIVERT TO SPEARFISH, SD. RADIO & RADAR CONTACT WERE LOST AT 1606. ON THE FOLLOWING AFTERNOON AT 1415, THE WRECKAGE WAS FND WHERE THE ACFT HAD COLLIDED WITH TREES ON A 4800' HIGH MTN AT ABOUT THE 4650' LEVEL. THERE WAS EVIDENCE THE ACFT IMPACTED IN A WINGS LEVEL, SLIGHTLY NOSE LOW ATTITUDE AT CRUISE AIRSPEED. THE COCKPITWAS DESTROYED BY IMPACT & GND FIRE. MEDICAL & TOXICOLOGICAL CHECKS REVEALED NO EVIDENCE OF PLT INCAPACITATION. THE ELEV TRIM WAS FND SET IN A SLIGHTLY NOSE-DOWN PSN.