Worn out yolk?

At the front, because the angle is constantly changing with changes in ride height, and sliding back and forth; it does tend to wear out the yoke..
But
3400 on the engine is never 3400 out the A904, unless you have installed a lock-up.
And so, by 3400 rpm, the driveshaft, at the front, is likely to be fully stabilized, unless if the driveshaft is too short; then it tends to flail about. Or if the angle is too shallow to properly lube the needle rollers, and they hammer out the U-joint trunnions.

I can also tell you that absolutely everything can be in perfect balance, yet if a tire is out of round or has a high-spot, or a heavy spot not properly compensated for, any one of those will still cause a vibration,once the tire is on the road;
as will a perfectly balanced but bent driveshaft, or one with improperly phased yoke saddles..
As will a rim not mounted hub-centric.

I once spent a great deal of time, chasing a small but annoying vibration. Off came the tires,out came the axles, then I dicked with the pinion angles, finally with nothing left, I sent the driveshaft away for the third time and when it came back, just a lil longer, it was almost quiet enough to ignore.
Finally; swapping out the 4.30s for 3.55s put the vibration out of cruising speed. It's still there, just at a much higher mph, where I almost never drive.

Good Luck man, I know your frustration.