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

I think redfish is on to something, up on hoist,likely not in neutral shop gave driveshaft a shake and determined no further investigation was necessary. Telltale will be wrench marks on nuts and bolts that hold shaft in, or lack thereof. Price of a good quality u-joint is money well spent for peace of mind. Then slipping ball/trunnion apart for inspection will at least help you rule it out.

Cant stress enough how important good safety stands are at this point. Good luck.