odd vibration

[Recian]

So I thought my worn out dryrotted tires were causing a vibration. At lower speeds 25-40 it intermittently kinda wobbles left to right if that makes any sense. It's an odd feeling, it doesn't feel like a balance issue. It starts up again at 60 and comes and goes over 60. This week I put new tires and switched out the wheels on it, road force balanced them. They were all .008-.023" road force. Spec for light truck on our machine is .036" so I know the tires and rims aren't the problem. I'm leaning toward a drive shaft. U-joints are tight. Trans fluid/filter could probably use a change. Smells old not burnt. Any other places I should look first? Any common areas?