I do not believe the reverse gear on NV3500s is synchronized. The internals may not have enough time to spin down before you shift into reverse, or it may be bad clutch disengagement like road trip was mentioning.
This is the test paraphrased from GM manuals for non-synchronized reverse gear.
Edit: this test is specifically for determining if your clutch is dragging, reverse gear is the most prominent detector of dragging because it is not synchronized.
1. Start the truck, push the clutch in.
2. While maintaining clutch in, shift to a forward gear
3. Still keeping the clutch in, shift directly to reverse.
Did reverse grind? Yes: Clutch is dragging bad, repair clutch assembly No: Continue
4. With engine running, shift to neutral, release clutch
5. Push clutch in, wait ~ 8 seconds
6. While maintaining clutch in, shift to reverse
Did reverse grind? Yes: Clutch is dragging slightly, adjust/repair clutch No: Clutch is functioning correctly
To note, it is NORMAL for reverse to be hard to engage/grind slightly/clunk if you go straight from neutral to reverse, it takes time for the disc/internals to come to a stop. If you quickly engage reverse from neutral the non-synchronized gears will grind.
The easiest fixes for a grinding but normal reverse:
1. wait for the trans internals to come to a stop with the clutch in before shifting top reverse (5-8 seconds).
-or-
2. Don't shift from neutral to reverse. Bump it into a forward gear to synchronize the transmission, then straight to reverse. Ex. Truck at a stop - Clutch in - 4th - Reverse - Clutch out (This is what I do). Always make sure you come to a complete stop or you will be synchronizing the transmission to a forward speed and exacerbating the problem.
