So my NV3500 manual transmission had been making a whining noise for 5-6 years in all gears except 4th. At one point in its early life, I believe the rear tailshaft seal had been leaking which led to low fluid and the eventual whine.
About a month ago, it started making a new noise: a very loud "clunk" that ran about 1/50th of the engine speed. So it was a very slow slunk but very loud. Rather than leaving me stranded. or worse, have it lock up while going 60 mph down the highway, I decided to tear it down.
I'll post pics for the benefit of others, or for those who are just curious