I searched the forum to see if anyone else has this issue. I found one entry that suggested the cab mounts need to be adjusted. I'm not sure how to determine if this is the root cause. Could the engine mount cause a similar effect if they are broken down? The link below has pictures inside the cab and the engine compartment.

http://picasaweb.google.com/erik.zollinger/Early1947GMC12TonLongBed#