Have you tried turning the tail shaft back around to compare.
re-check unis feel smooth still, they can vib a bit.
when you swapped tail shaft around they can be reassembled out of line and this also can cause a vibration.

sometime the tail shaft angle can be to much for the unis to handle whilst turning and they vibrate as a result.
usually over 2" lift for said angle, but ya never know.

I some times get a vibration that sounds a bit like what you describe, but it is on a certain bit of RD only.
small ripples in the asphalt I recon, never happens anywhere else though.

speedo problem, transfer cases can have different drive gear tooth count, might be your problem.
there's a thread on here about, talking on this subject, different drive gears incl part numbers.