I'd also vote for the center bearing or support as you have pretty much eliminated the normal suspects. My recent parts car acquisition had a vibration going forward and a huge amount of noise and vibration in reverse. Was told the "diff fluid needed changing"
. I didn't care as I wanted the 5 spd bits. Got it home, drove it in reverse, helper said noise was from the middle. The center bearing was fine, but its rubber support was almost non-existent. Its not a difficult job, just a whole lot of R+R to get the shaft out.