It's crazy to me the amount of driveline shops I've talked with that never even heard of critical speed before.

I've seen trucks come in with amazingly long 3" steel driveshafts that would have blown apart at 5k rpm. So, it is possible.

I would have a driveshaft built with at least a front CV rather than a u-joint as I would guess it would end up with a lot of working angle on the front joint? Unless the tunnel was rebuilt to bring the angles closer to zero.