Mystery vibration at 25 mph can't be diagnosed. It's not the front end!

Thanks for all the information. I live in a one horse town, so I have to lean on this shop for my projects. Problem is that hardly anyone is over 30 years old and they're not used to older vehicles. I'm going in on Monday and talk to the foreman about engine & trans mount and B&T inspection. I want to make sure they understand this is the probable problem.