I feel your pain. I just went thru 2 small blocks 318 and 360 that did the same in my 84 d150. The only reason I could come up with as to a cause is the torque converter "balloning" when I was pulling a load. I was able to find a replacement crank cheap in both cases.I did talk to a crank grinding place about it and they said that the cranks could be welded and resurfaced so if yours is a steel crank it may be a way to recover it.