That's tough to say. I've personally not had any experience caused by vibration from worn u-joints, it's usually been from driveshaft damage or a balancing weight getting knocked off. If your mileage is up there and the zerks on the driveline haven't been greased regularly then it's a small price to pay for knowing they're new and you'll maintain them going forward. And if you still have the vibration then that part is eliminated.

One thought: Have you gone down the length of the driveshaft from u-joint to u-joint and given it a good visual inspection?