PRIOR TO SEPARTING, THE PLT WAS ADVISED OF LOW CEILINGS & POOR VISIBILITY OVER THE ENTIRE PROPOSED ROUTE & WAS ADVISED THAT VFR FLT WAS NOT RECOMMENDED. IN A DISCUSSION WITH PERSONS AT FERGUS FALLS, MN, HE STATED THAT HE WAS GOING TO FOLLOW THE HIGHWAY. ABOUT 20 TO 25 MIN AFTER DEPARTING FERGUS FALLS, THE ACFT CRASHED IN A BARNYARD ON LEVEL TERRAIN. A WITNESS STATED THAT BEFORE THE PLANE CRASHED, THE ENG SOUNDED LIKE THE ACFT WAS PERFORMING DIVES & STALLS & THAT THE 'POWER WOULD INCREASE AND THEN SLOW DOWN.' HE THEN SAW THE ACFT COME OUT OF THE OVERCAST AT A HIGH RATE OF SPEED IN ABOUT A 45 DEG NOSE DOWN, 70 TO 80 DEG RIGHT WING LOW ATTITUDE. HE ESTIMATED THAT HE SAW THE ACFT ABOUT 2 SECONDS FROM THE TIME IT CAME OUT OF THE CLOUDS UNTIL IT IMPACTED THE GROUND. NO PREIMPACT, MECHANICAL PART MALFUNCTION OR FAILURE WAS FOUND.