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

I will tell them to check B&T again on Monday and definitely tie all engine and trans mounts down tight. It seems like such an easy fix, but they're very busy and probably just gave it a quick glance. This is crazy! For this problem to just come out of thin air seems to point to something came loose. I broke my back, so I can't do it myself.