The NV3500 is a different animal then the NV4500 (front load vs top load), not sure its subject to the 5th gear nut issue like its bigger brother, I have no real experience with one though. Is this a long wheel base truck? carrier bearing comes to mind. Chock the front wheels jack up the rear and and spin/shake the driveline, should find a faulty u joint, carrier and possibly even the mentioned diff issue if its there.

I know when my buddy lost 5th gear mnay years ago it started as just a ticking noise that slowly got worse

 

It doesn't have a carrier bearing. I have a front and a rear driveshaft. Two piece.

 

I have the same truck.

Would like to see the video you made. Can you post it on youtube?

I've had my share of vibes/noises over the years (owned since 2001) and this is what they were:

1. clunk under floorboard = shock bolts
2. whine or squeak when turning = front ujoints
3. rotational squeak in reverse = drive shaft ujoint
4. rotational grinding noise in front = stone in caliper
5. serious vibes when braking/decelerating = tires
6. vibes when braking = brakes
7. vibes at 75mph+ = tires/bearings/exhaust
8. vibes at 30+ = exhaust drone
9. vibes in brake pedal = tires and/or exhaust
10. clunk when turning = ujoints or bearings

I love the NV3500 and it's a completely different animal than the 4500/5600. It's a 2-piece aluminum body and has no known quirks or issues, save to use the proper fluid. The 5th gear nut is not an issue. Mine has 285k on it and I've used it hard over the years with lots of towing and hard miles. I highly doubt it has any problem unless the PO never serviced it. You can get brand new rebuilds from Midwest Transmission. It is one of the stronger manual trans made in the 90s and carries the stereotypical 2nd gear whine that denotes stronger gears.

If you haven't changed ujoints or bearings recently, do them all. Use only Spicer brand as they are OEM and last the longest. I'm on my 4th set of Spicers in 22 years. I have all the part numbers if you need. Bearings use Timken if possible. Rock auto has the best prices anywhere. I've done a lot of repairs and maintenance on my truck and have seen nearly every issue except the two that everyone said would happen: head cracking and rear end blowing up. All still stock.

Happy trucking!

 

This is the video I made with the noise at 77mph.
https://www.youtube.com/shorts/zH1h7mDycds

 

It doesn't have a carrier bearing. I have a front and a rear driveshaft. Two piece.

 

https://www.youtube.com/shorts/SD8XwCuBkYM

 

Actually pretty good vids but I don't have great suggestions. It could be the rear end.....the 9.25 is good but does fail at high mileage sometimes. However the one I experienced was more of a consistent deep howl, not cyclical.

In the one vid you zoomed in on the t-case lever and that reminded me of motor mounts. Power brake it and watch that shift lever. If it moves left/right much you have a toasted motor mount. NOW, would that cause your noise at 77? Eh, maybe.... but I wouldn't say likely

If you're getting bucking in 3rd and 5th that sounds like a misfire, but also wouldn't cause a rotational driveline noise.

 

That noise doesn't sound like anything bearing or ujoint related. It's too slow. Perhaps tire drone. What tires do you have? Certain treads can make so much noise/vibe it makes a guy insane.

Could be the t-case bearings and the chain is whirring loosely. Tested it in 4wd? You can go 70 on a nice straight road in 4hi.

Also doesn't sound like clutch. Could be bearings in trans. Pop off the rubber boot over the shifter and listen there. A LOT of sound is kept from the cab from that boot seal.

What happens in each gear? Is the noise only at 75+? What about 4th gear at 75+?

 

Transmission shifts smooth. It goes into 1,2,3,4,5 all fine. It's just a thumping noise underneath. the lever just vibrates when i shift to early on the rpm. 4X4 works 4hi and 4lo. I drive in 2hi all the time as your supposed to.

 

If you shift into neutral at speed is the noise still there? Does throttle position make any difference in the noise(when in gear)?