Not sure and all this guessing is not helping you, I realize that. I will stick with guesses, theories and the fact that it needs to get worse to find. BUT............. I offer another bit to ponder:
The slip yoke has splines, kind of like gear teeth. The ring and pinion do have gear teeth. The trans has gear teeth. In general when these are "driving" they are in tight mesh, so showing them clunking back and forth in static mode does not mean that the "looseness" is making the noise. Probably they are not "loose" during torque. The ring and pinion teeth are cut to not make noise like a spur gear does, but they can make that noise if the mesh or the backlash it wrong due to wear. That noise can translate to the front cab area. But who knows? It is still a mystery as to gear or bearing noise or vibration. I did not notice if you did a series of coasting tests yet. With clutch pedal depressed. Coasting is a good diagnostic because there is no drive torque on gears and clutch is disengaged. Sometimes it may give clues. More experimentation is advised.
The questions are is this speed related? what gear ratio it's in at the time? Does clutch in and out change anything? Does coasting give a clue? What area is the noise actually coming from? Is it gear or tire noise? (it can be tire noise. it can be wheel bearing noise, it can be a clutch fan if you have one). Is it vibration or mechanical rotation noise? Who knows? So there is your really helpful guess again
