Growling 3rd and 4th 13 speed transmission

I have a hard time seeing it being a synchro. Its locked up all the time until you change between hi and lo range. If a synchro was making noise I think you'd hear it only during a range shift.

If its only doing it on the low side I'm guessing a bearing or gear in the aux box.

 

Unless you are running long I would be slow to jump into spending big money. Play with the ride height. Stick a magnet in the diffs. Shoot the u joints with a heat gun when you come off the road. Just thanking out loud.
Check engine mounts.

 

If it’s only when you let off triple check that ride height. Bearings are all time noise if it’s turning, gears whine under load, sliding clutches and synchronizers jump out or growl along with forks. 3 and 4th are the first 2 behind the input shaft. Make sure your input shaft Or attached pieces ain’t involved.


Like @BoxCarKidd said look at diffs etc. I’ve seen lots and lots of trans missions replaced that ended up being a bad rear or worse ride height