AT 2359 THE LOCAL CONTROLLER CLEARED THE FLT TO LAND ON RWY 6R. AT 0001 THE GROUND CONTROLLER CLEARED A STATE-OPERATED PICK-UP TRUCK ONTO RWY 6R FOR A TAPLEY RUN. THE GROUND CONTROLLER STATED THAT HE REQUESTED CLEARANCE FROM THE LOCAL CONTROLLER TO ALLOW THE TRUCK ON THE RWY. HE WAS NOT AWARE THAT THE B-747 HAD BEEN CLEARED TO LAND. THE LOCAL CONTROLLERWAS BUSY WITH OTHER COMMUNICATIONS & WAS NOT SURE IF HE ACKNOWLEDGED THE REQUEST FROM GROUND CONTROL; HOWEVER, THE GROUND CONTROLLER BELIEVED THE LOCAL CONTROLLER SAID 'OKAY.' THE ACFT STRUCK THE TRUCK ABOUT 2,000 FT BEYOND THE APPROACH END OF THE RWY WHILE THE ACFT'S MAIN LANDING GEAR WAS ON THE GROUND BUT THE NOSE HAD NOT YET BEEN LOWERED FROM THE LANDING ATTITUDE. THE FLT CREW STATED THAT THEY DID NOT SEE THE TRUCK PRIOR TO THE COLLISION. AT 0013 THE RWY VISUALRANGE(RVR) WAS 600 FT VARIABLE 800 FT.