THE PILOT REPORTED THAT HIS FLAGMAN'S PICKUP TRUCK BLOCKED HIS VIEW OF A RAILROAD CROSSING SIGN. DURING WHAT WAS TO HAVE BEEN THE FINAL SWATH RUN, THE TRUCK WAS MOVED REVEALING THE SIGN. THE PILOT ATTEMPTED TO PULL UP, BUT WAS NOT ABLE TO AVOID COLLIDING WITH THE SIGN. THE AIRPLANE SUBSEQUENTLY IMPACTED UPRIGHT IN SOFT TERRAIN AND CAME TO REST ABOUT 100 FEET FROM THE SIGN.