My old die grinder seems to run smooth as silk when there is no bit / burr in it.

As soon as I put in a burr and turn it on, it has an out-of-round vibration. It's almost not enough to see with your eye
but you can hear it and feel it, and when you try to put it to a surface to grind on, it jumps around in an unsafe manner.

The burr has a short 2-in shank and the diameter of the cutter head is only about 1/2 inch.

So the vibration must be from the burr. It was $23 twenty years ago and is brand new / undamaged.

Are the TIR tolerances that tight on a 26,000 RPM cutting burr ?

Where do you buy quality burrs these days that don't vibrate ?