Vibration only in 5th gear (manual transmission)

336,000 miles, 1998 5 speed manual. I’ve been chasing a vibration at highway speeds that feels like it’s in the rear end. I bought a Tom Woods driveshaft thinking that is the problem. That took some of the vibration away but not all.It definitely made the shifts much smoother. No clunking. I’ve driven the truck in 4H with the rear driveshaft out of the truck. Runs smoothly on the front driveshaft. Even in 5th gear. But I still have a vibration that only happens in 5th gear with the rear driveshaft in the truck in 2H. I can run 65 mph in 4th gear and there is no vibration. Shift in to 5th gear and it is the subwoofer sound that lots of people complain about. It happens at lower speeds in 5th gear starting at about 50 mph. I can roll the window down and it’s not as bad but I can’t always ride that way. Anyone else have this problem? What was your fix? Bearing in the transmission or transfer case?