The pilot stated that while maneuvering, he felt a severe vibration. He dumped the chemical and during the forced landing, collided with trees before coming to rest. Examination of the aircraft revealed that the No. 2 prop blade counterweight bearing shaft failed due to fatigue. According to the metallurgist, the fatigue was due to bending stress which was caused by excessive clearance between the shaft and the bushed hole it was positioned in. This allowed centrifugal force to position the blade to high pitch, resulting in unequal thrust. The propeller was overhauled in 1987 but the total time since that time could not be determined. The propeller was repaired about 10 weeks prior to the accident but had only accumulated 3.4 hours since the repair. According to the prop overhaul facility general manager, the propeller was returned to the operator after the accident but the approved for service line on the work order was not signed. According to the chief inspector of the repair facility, further inspection was necessary to determine if the propeller was airworthy.