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

I've never seen weights fall off a converter. I have seen a lot of vibration at a speed be it 25 mph, 35 mph, 60 mph, whatever,,, and it has usually been driveline components like U joints. I remember one chevy work van that belonged to a TV repair shop across the street from this shop. The owner complained of a vibration right at 50 mph, not 45, not 55. I never felt it during the test ride, even when he asked, "Did you feel that?".
I took the driveshaft out and felt no problem working the U joints around in hand. My shop owner/bossman said, "I already sent Jake after new joints for it. Hell, Blanton's got money". So during my taking the joints out... WHOOT there it is! Just 1 of the 4 little shafts marked up pretty good. Then the boss said something like "He knows his van like he knows his wife. The customer is always right."
My point being,,, Eventually the fault will get bad enough that anyone could easily diagnose. So keep hunting and guessing or just drive it waiting for the fault to truly shows itself? That's your decision.
Heavier duty springs have a different sag rate but shouldn't make a difference in normal ride height. Maybe the 6 leaf springs were re-arched or otherwise not correct for your vehicle.