Growling under load in 4th, AX15

[asymptonic]

So I've had my XJ in and out of a transmission shop totaling about 6 weeks over the last three months due to a growl whenever I put the Jeep under load, eg hard acceleration or mild hill climbing in 4th gear around 40mph. Maybe audible in 5th but really bad in 4th. A lower than engine RPM growl in this case.
They dropped the transmission and inspected it and found "nothing wrong", which isn't wrong, no broken teeth etc. Had it looked at 5y ago with new synchronizers being the only needed work. After test driving it with them the guy is pretty sure it's normal wear causing some gear lash in that gear.
Just want to get the general opinion of y'all on whether that tracks. I have the money to work on it for once and am a stickler for staying ahead of problems, noises etc.
Other things that have been checked out: new u-joints in the last 6 months, in and out of the rear diff a couple times due to a mysterious brake issue so probably okay there. New clutch a couple years ago.

 

[asymptonic]

Following up, 4th is the 1:1 gear? What parts of the transmission are even involved in that gear? What could wear out? They did replace the pilot bearing and the throwout bearing has been replaced twice in the last 6-7 years afaik.

 

[lawsoncl]

Yes. 4th is the 1:1 ratio.

 

[RCP Phx]

I would drop the rear driveshaft and drive it to "isolate" the area that's causing the noise!