Combines Vibration on 8820

This I can answer,yes it is the tires.try to get above that speed as soon as you can and it should stop also try a little more air in the tires,it will still shake but maybe at different speed.If all else fails turn the tires around backwards.

I once had this same kind of shake in my 81 8820. It turned out to be the left final drive. This particular machine had planetary type finals and the outside hub bearing went out and the planet gears during each revolution rubbed on the housing. It was usually fine until 4 mph. If this doesnt seem to be the problem, check all drive shafts from trans to final. Good luck. let me know what you find.