Mike,

That's what they do, weld small tabs of metal to the shaft, normally at the ends. I would think any which had come off would show up as rough spots on the shaft where the welds were broken as well as leaving spots where the paint on the driveshaft was "different" looking. Other than scraping them on a rock or something I dont see how the weights would ever come off though.

Mine were obviously bad. You might get under the truck and twist the driveshaft by hand while looking at the yolks. if you can see any movement between the u joints and the caps that go into the shaft or yolks the joints need swapped.

I tried doing them myself but never have I seen ujoints so tough to remove. I took both the front and rear shafts and new joints to a driveshaft shop. They swapped and balanced both shafts for 60.00.

In my truck it made no difference in the vibration but they did need changed.