NP 435 trans help

If the nut is tight holding the trans flange, then the play is probably in the slip yoke. They wear out eventually especially if you don't get regular grease jobs. They fit all kinds of Dodge trucks and vans so a replacement should be easy to find.....either new or used. Sometimes it's the splines on the driveshaft that wears out. In that case you need a new driveshaft. A little more money but still should be easy enough to find. I've never actually seen one strip so I'd run it until the vibration gets bad and then change it.
Thanks, Mark