Joe, here's something to keep in mind (no matter whether you're looking at transmission or rear end gear noises). When you're accelerating, force to the gears is being applied in one specific direction. When you are decelerating, force is being applied in the opposite direction. This I realize does not answer any questions per se, but it does give you a hint. Long use of an an old transmission or rear end causes gears and shafts to develop very distinct wear patterns. It might be tempting to say "well, acceleration wear and deceleration wear will more or less even out", but in my experience acceleration wear is the more severe of the two...for what I think is an obvious reason. More constant applied force/resistance and all. Anyhow, listen carefully and see if maybe you can remove the transmission cover, get down on the floorboard and use one of the cheap stethoscope things to listen better as another person drives. Just be careful doing that.
Also in my experience and not many seem to catch this...I have seen several of these old trucks where the cab is sitting right straight on the frame. Very common as I said. What happens is over a span of 70 or so years the rubber pads compress to the point where there is no more noise isolation between the cab and the frame. When that happens, any vibration or noise coming from the frame (and there will be plenty) goes into the cab where the cab acts exactly like the body of a guitar and amplifies it. How to fix? Two ways you can do this. Loosen the front end and remove the nuts from the two bolts holding the front of the cab to the frame, jack up one side of the front of the cab a wee bit and slip pieces of pvc shower liner material cut square with a U shaped center cut until you can see just a little slit between the cab mount and the frame when you lower that side of the cab and replace the nuts on the two bolts and tighten them when you're through. Or remove the bolts one at a time and replace the rubber pads with new ones. Then tighten the front end and you should have improved things. Point I'm making here is that your old transmission or differential might be making normal noises for their age, etc but you may be hearing them more loudly than you need to. These trucks were never really engineered to be whisper quiet but they were pretty nice when they were new. Good luck!
