Their were some lubricating issues with the Hi-Low range planetary on the rear of the transmission with some early models. The spider pinions were not receiving enough oil to maintain a long life. The noise should change or stop from shifting from low to high, if this is an issue. A rather large nut holds the tran countershaft in place. If the nut comes loose, end play will develop in the countershaft. I would think the noise would be more in the tran. The nut is staked to hold it in place. If the tractor rear tires were spun alot, the siper pinion gears in the diff would run out of lubrication causing them to wear prematurely. These were main issues for these tractor over the years. The diff lock assy is on the right side of the rear pinion. See if pushing the pedal changes anything. Make sure the pedal is raising to disengage it.